It’s not about when you attack but how.
Focus - Focus - Focus. I like to attack from the south and secure the factory in Ukraine. If you build a minor in Romania, by a build of 2 arm, 1 mec, + 3 inf you can effectively crank 6 units a turn into Ukraine. If you have left over aircraft - which you should, you can build your stack in such a way that the Russians can’t beat it.
A counter Russian focus on your Ukranian stack, will have them giving up Leningrad.
The best way to beat the Russians, is to have them attack YOU, in positions where they can’t win. Whilst you lie in wait for an Italian aircraft can-opener manuever, for a strike at the captial. Erosion is the name of the game.