If Germany is played by a human player , never .
They simply loose too much of their army (even though not significantly more than in RL) in the first 2 - 3 months .
By loading saved games and looking what they were doing I noticed that they go very early in a panic mode and overload their TC with strategical deployments .
The thing is that they are sending their armies to places that will fall before the armies get there .
So they are basically in a permanent state of unpreparedness from which they can't recover .
The one thing that can save SU is US going in war very early and threaten France .
The only thing that looks non realistic are the minors .
Hungary , Romania , Slovakia , Bulgary , Finland and liberated Ukraine provide so many units that the concentrated Wehrmacht is unstoppable .
A german player (human or AI) can easily leave the southern half of the front to his allies and only bother about the northern half .
Once he gets Leningrad and Moscow , he swings south and it's game over for the Soviets .
If in RL Germany had total military control of its allies as well as of Italy like in the game then the SU wouldn't probably stand a chance either .