When I say "realistically," i mean by if the allied forces don't suddenly turn into blithering morons with an army of cowards.
When, on 11 December 1941, Germany declared war on the USA. The vast ability of the USA to manufacture war goods was now able to assist not only their own military but that of the USSR and the British Empire. By December 1941 Germany was already relearning an old lesson in Russia but their astonishing decision to declare war on USA made the end only a question of time.