How long could World War Two last, but with still the same result? i.e. the Axis loses, the Allies win.
Barring a PoD in the 1930s that lets Nazi Germany develop nuclear weapons in 1945, they are crushed by American nukes in 1946. Germany could certainly wage Barbarossa in a better way and achieve a Brest Litovsk partial victory, or at least a white peace draw with the reestablishment of 1941 borders. And use the freed resources to repel any Allied conventional landing in Europe.
But they cannot escape the American nukes that are coming in 1946 at the latest. There is a little possibility that they could establish an effective MAD with the Western Allies by threatening Britain with V-2 with dirty-bomb or nerve gas warheads, and that is their only hope of survival, but surely it's not the most likely outcome.
It is conventional wisdom on this board that WWII cannot last beyond 1945 because of American atomic bombs. But the development of atomic weapons as IOTL is not a given- any number of PODs could delay their development for a long period of time or conceivably butterfly them away entirely.
Even if the Manhatten Project falls through or if the Germans still perform better on the Eastern Front, I think the end result is still an Allied victory. None of the Axis members had the resources for a long-term war (which was a big reason why some of them took the actions they did), so as long as the Allies could keep outproducing them, the Axis are guaranteed to lose.
In response to people saying that US nukes automatically end the war, is there any POD (or series of PODs) that gives Germany air dominance over Germany with a Ta-183 derived interceptor with a service ceiling of 50,000 feet by August 1945?