One of the real problems folks have when they discuss Generalplan Ost is that that there was never a "finalized" version with all the proper signatures and Fuhrer Directive Number. This wasn't because the Reich had any intention of not carrying out the Plan, but because it was ever evolving as the war waxed and waned.
However, what is more or less indisputable is that the goal was to effectively obliterate the Polish people, leaving a remnant illiterate slave workforce to labor on the "settler plantations" that Hitler envisions gifting to senior Wehrmacht officers and Party loyalists, and to serve as helpers on the smaller farms that would have been granted to enlisted veterans (yes, and the enduring mystery is how he expected life long, 2rd or 3rd generation city dwellers to become farmers, happily working the land). This same group of ex-Wehrmacht officer and Landsers would also serve as a permanent militia force, prepared to deal with any sort of uprisings among the slave population (I rather suspect he stole this straight from the Escaped Slave Posse idea that existed across the Antebellum South), freeing the active force for serious border issues and to prepare for future conquests (Hitler believed that a country was destined to continue to expand or perish).
There is also a revisionist effort centered on the idea that the Reich would never have actually have even seriously tried to complete Gerneralplan Ost. This ignores the basic reality that the REich leadership was, in many ways, patently insane. While there would certainly have been efforts by various Gauleiter to preserve more of the population under their control while forcing competing regions to reduce more (primarily because higher production = more opportunity to embezzle, most of the Reich leadership were corrupt to a shocking degree), in the end it wouldn't matter. There enough true believers, starting with Himmler and Goebbels, with enough minions who were equally invested in the Cause, that any effort to fudge the numbers would have, overall, failed.
There is also something of a tendency for many modern commentators to believe that the German population itself would not have allowed this sort of genocide, that the common German Landser simply wouldn't have executed mass numbers of innocents, especially children. The difficulty with this is that the actual historic record illustrates that this is simply untrue. Certainly many Heer personnel would not have willingly shot down masses of civilians, however, there would not have been any need for them to do so. All Heer personnel would have been required to do was maintain a perimeter, not conduct the actual killing. A reasonably limited number of SS and Einsatzgruppen personnel would be, horrifically, sufficient to complete 90% or more of the Plan. IOTL the Einsatzengruppen never far exceeded 3,000 members. Despite that, they are considered to be responsible for at MINIMUM of 1.5 MILLION murders, perhaps 2M. We are all aware of how a very limited number of SS personnel managed to murder millions in a limited number of extermination camps, and slightly larger number of "Extermination through Labor" camps, the overall Generalplan Ost effort would have required little more field work, but not an exceptionally larger personnel allocation (Waffen SS divisions would have easily proved personnel in sufficient numbers without damaging the Reich's military position, given the reality that this entire scenario presupposed the defeat of the USSR, freeing up roughly 2/3 of OTL entire Wehrmacht).
The biggest, and easiest, way to implement "Extermination through Labor" is well known. Hitler, in his grandiose vision, required that all "Slavic" cultural traces be literally obliterated (book, art, music, etc) and that "Slavic" major cities (Warsaw, Kiev, Moscow, Saint Peterburg/Leningrad, etc) be literally destroyed until no two stones stood together. How could this be even remotely possible? By using the population of the city and outlying districts to disassemble the entire city, mainly by hand, mostly with improvised tools, working from dawn to dusk, while receiving 4-600 calories a day, no medical care, and minimal shelter. It doesn't take much imagination to realize just how rapidly the population would be destroyed in this circumstance (it is almost a certainty that they sort of mass crematoria featured in the camps would need to be constructed on-site, simply to manage all the bodies). It is a near certainty that the Reich would have run out of bodies before they ran out of Moscow, much less the mad plan to turn Metro St. Petersburg/Leningrad into a massive lake. Other parts of the population, including those with the most useful skills in the "trades" would have been deported to Berlin to reconstruct the Capital of the Reich into Germainia (about as serious a Fool's Errand as one could imagine in the area of Urban Planning). They would have been fed slightly better, but the death rate, even among these artisans and expert slaves would have hovered around 30% annually.
Lot of set up to answer the OP's question - The recovery time is entirely dependent on how long the Reich survives. It is unlikely to have the same 70 year lifespan as the Soviet Union, the Soviets at least accepted the utility of education and the scientific method, and corruption was on a somewhat smaller scale. The Reich was, in addition to being run by a pack of evil bastards, also enormously inefficient and rather hostile to higher education if it did not adhere to Party Doctrine. If it collapsed in 40 years (i.e. ~1973), there might be a chance of some sort of Slavic society/culture to survive, although it is questionable how much of it would be non-Russian, since the Russian Slavic population would be the overwhelming majority. If, however, the Nazi system survived for the USSR's three score and ten while retaining control of the European Peninsula to the Urals, there wouldn't be enough Slavs in Europe to repopulate the East. What would emerge would be a bastardized version of the various Slavic traditions as kept alive by the global diaspora of the various countries (primarily from North America and the UK/Commonwealth) and traditional German/Russian/Finnish/Scandinavian influences.
However, what is more or less indisputable is that the goal was to effectively obliterate the Polish people, leaving a remnant illiterate slave workforce to labor on the "settler plantations" that Hitler envisions gifting to senior Wehrmacht officers and Party loyalists, and to serve as helpers on the smaller farms that would have been granted to enlisted veterans (yes, and the enduring mystery is how he expected life long, 2rd or 3rd generation city dwellers to become farmers, happily working the land). This same group of ex-Wehrmacht officer and Landsers would also serve as a permanent militia force, prepared to deal with any sort of uprisings among the slave population (I rather suspect he stole this straight from the Escaped Slave Posse idea that existed across the Antebellum South), freeing the active force for serious border issues and to prepare for future conquests (Hitler believed that a country was destined to continue to expand or perish).
There is also a revisionist effort centered on the idea that the Reich would never have actually have even seriously tried to complete Gerneralplan Ost. This ignores the basic reality that the REich leadership was, in many ways, patently insane. While there would certainly have been efforts by various Gauleiter to preserve more of the population under their control while forcing competing regions to reduce more (primarily because higher production = more opportunity to embezzle, most of the Reich leadership were corrupt to a shocking degree), in the end it wouldn't matter. There enough true believers, starting with Himmler and Goebbels, with enough minions who were equally invested in the Cause, that any effort to fudge the numbers would have, overall, failed.
There is also something of a tendency for many modern commentators to believe that the German population itself would not have allowed this sort of genocide, that the common German Landser simply wouldn't have executed mass numbers of innocents, especially children. The difficulty with this is that the actual historic record illustrates that this is simply untrue. Certainly many Heer personnel would not have willingly shot down masses of civilians, however, there would not have been any need for them to do so. All Heer personnel would have been required to do was maintain a perimeter, not conduct the actual killing. A reasonably limited number of SS and Einsatzgruppen personnel would be, horrifically, sufficient to complete 90% or more of the Plan. IOTL the Einsatzengruppen never far exceeded 3,000 members. Despite that, they are considered to be responsible for at MINIMUM of 1.5 MILLION murders, perhaps 2M. We are all aware of how a very limited number of SS personnel managed to murder millions in a limited number of extermination camps, and slightly larger number of "Extermination through Labor" camps, the overall Generalplan Ost effort would have required little more field work, but not an exceptionally larger personnel allocation (Waffen SS divisions would have easily proved personnel in sufficient numbers without damaging the Reich's military position, given the reality that this entire scenario presupposed the defeat of the USSR, freeing up roughly 2/3 of OTL entire Wehrmacht).
The biggest, and easiest, way to implement "Extermination through Labor" is well known. Hitler, in his grandiose vision, required that all "Slavic" cultural traces be literally obliterated (book, art, music, etc) and that "Slavic" major cities (Warsaw, Kiev, Moscow, Saint Peterburg/Leningrad, etc) be literally destroyed until no two stones stood together. How could this be even remotely possible? By using the population of the city and outlying districts to disassemble the entire city, mainly by hand, mostly with improvised tools, working from dawn to dusk, while receiving 4-600 calories a day, no medical care, and minimal shelter. It doesn't take much imagination to realize just how rapidly the population would be destroyed in this circumstance (it is almost a certainty that they sort of mass crematoria featured in the camps would need to be constructed on-site, simply to manage all the bodies). It is a near certainty that the Reich would have run out of bodies before they ran out of Moscow, much less the mad plan to turn Metro St. Petersburg/Leningrad into a massive lake. Other parts of the population, including those with the most useful skills in the "trades" would have been deported to Berlin to reconstruct the Capital of the Reich into Germainia (about as serious a Fool's Errand as one could imagine in the area of Urban Planning). They would have been fed slightly better, but the death rate, even among these artisans and expert slaves would have hovered around 30% annually.
Lot of set up to answer the OP's question - The recovery time is entirely dependent on how long the Reich survives. It is unlikely to have the same 70 year lifespan as the Soviet Union, the Soviets at least accepted the utility of education and the scientific method, and corruption was on a somewhat smaller scale. The Reich was, in addition to being run by a pack of evil bastards, also enormously inefficient and rather hostile to higher education if it did not adhere to Party Doctrine. If it collapsed in 40 years (i.e. ~1973), there might be a chance of some sort of Slavic society/culture to survive, although it is questionable how much of it would be non-Russian, since the Russian Slavic population would be the overwhelming majority. If, however, the Nazi system survived for the USSR's three score and ten while retaining control of the European Peninsula to the Urals, there wouldn't be enough Slavs in Europe to repopulate the East. What would emerge would be a bastardized version of the various Slavic traditions as kept alive by the global diaspora of the various countries (primarily from North America and the UK/Commonwealth) and traditional German/Russian/Finnish/Scandinavian influences.
