Give or take, a few months. The Battle of the Bulge delayed the Western Allies advance so...It depends on how those forces are used "Defensively".
Hitler has the following choices he can make:
A. Send them off to stem the Red Tide.
B. Keep them in the Western Front to try and create a stalemate.
C. Send them off to Italy or the Balkans.
D. Use them to bolster homeland Germanys forces, waiting for the invaders.
E. All of the Above.
In any case, Post-War Germany is going to be different, depending on the choices. Using them in the West will create a similar to OTL with minor butterflys, using them in the East will give the Western Allies more bargining chips. I'm more interested in how he would use them in Italy a/o the Balkans. Using them in occupied Yugoslavia could somewhat reassure Croatia that Germany was a proper ally; The Serbian-Croatian conflict would become much more deadly in the time to come.
And to make my points understandable, playing defensive will only create some minor butterflys. Germany is doomed at this point and Roosevelt still trusts his, 'best friend' Josef Stalin so you can't expect more then that.