jahenders
Banned
I think Stalin would wait to build up somewhat and then watch for opportunity WRT Germany. If/when Germany stumbles of is too heavily engaged, Stalin would make some initial thrusts (perhaps more of Poland).
I think, based on what little I know it would take a combination of circumstances for Stalin to invade Germany on his own.
If I remember correctly, the Red Army had begun a 5 year plan in '37 which was slightly derailed by the Winter War, however the main part was still on course to be complete around 1942. At that point, the German high command would have a good idea at that point that they probably couldn't defeat the Soviets as they could simply be swamped by a seemingly endless pool of manpower*.
I think it would need the completion of the 5 year plan AND some instability in Germany, be it Hitler's death, a coup, financial issues, Tube Alloys anything. Germany doesn't need to collapse, just get thrown slightly off balance. At which point I could see a push for a good chunk of the remaining Poland, ("liberating our fellow Slavs..."), and potentially make a good stab at Finland into the bargain.
The end result is a bloody stalemate until quantity eventually wins out, at which point France, the Benelux countries and Britain will be worried.
*it wasn't, but still...