How long would it take for Rommel to complete the Wall to the above specifications?
Once again, it depends. What is the deal with the USSR?
Status quo antebellum, including the same shipments of materials? Does the Reich get all of Poland? Does it get increased shipments of materials? Did it gain additional territory? Baltic States? Western Ukraine? Territorial gain provide more forced workers.
The answer to the time it will take is entirely tied up in those questions? If the Reich has access to significant resources out of the USSR, then it can do much more, and much more quickly, than if it is getting somewhere near the pre-war shipment level.
If the resources are available it then becomes a matter of moving forced labor from Poland to supplement the 600,000+ French workers conscripted by the Vichy IOTL. Adding workers obviously increases production as long as supply is available. Concrete isn't difficult to produce in bulk (Portland cement does not use anything that could be seen as rare or subject to shortages due to the WAllied blockade) and often makes use of what is seen as waste from other industrial processes) Steel for reinforcement is one of the easiest things for a plant to manufacture, Soviet shipments could readily include tons of rebar even in a nearly status quo peace deal, Soviet plants were producing a great deal of steel, even without Lend Lease. The biggest limitation would be draft horses.
If you take two million forced laborers (IOTL it approached 1M a couple times, but was usually closer to 350K) and use the frontage I discussed earlier, you are talking around 3,000 laborers per mile. probably 2/3 of them are involved in prep work, rock crushing for aggregate, cutting rebar, felling trees, making forms, otherwise getting the actual materials in ready condition and moved. That leave 1,000 people to do the construction manual labor, 110 people can readily dig a 1,500 meter trench in a week (that is one two meter deep, two meter long and one meter wide hole a day, per person, so I am going WAY low on productivity), 400 can position prefabricated forms and rebar over the same distance in a week, the rest can mix and pour the concrete. (It is hard labor, and these are not going to be the ideal workers, so keep that in mind when you realize who low the overall productivity is). That gets five miles square of concrete lined trenches,space 400 meters apart with lateral communication trenches in around 18 weeks. Adding a series of combination mg positions and shelters from bombing/shelling is another 12 weeks. Heavy pillboxes for machine guns, light AT/anti-boat guns partly dug in so only about two feet of the bunker is visible, with overburden added back over the top, figure 50 laborers for three days or 20 or so a week, 10 for bigger, more elaborate set ups for heavy guns. In a year you have a stretch of fortification from Brest to Northern Holland with trenches, dugouts, and 300-400 bunkers in each five miles square in a year. Of course these are averages, in some areas it will be easier, others much more difficult, and the time frame can be compressed with additional personnel (and horses, don't forget the horses) and sped even further if any significant amount of earth moving equipment is available.
There can be some interdiction of the transport, but that means less effort in the strategic offensive against Germany, and it took a good deal of experimentation by the USAAF to get a really useful tactical application that interdicted transport, and it only really worked once the Luftwaffe was defeated, something that is going to considerably more difficult in this scenario.
