I will usually wait until G4 to attack Russia. Yeah, they will have more stuff to fight, but Germany will have even more and probably better stuff (tanks & planes vs infantry).
Sometimes this can lull the Russian player into a false sense of security and he will buy more expensive items instead of all infantry on one or two rounds.
Main reason for waiting this long is trying to get Sealion out of the way, or at least killing off the Royal Navy so UK is trapped on their island. Also, it takes some time to get all your firepower staged on the Russian border for a proper invasion. I like to go into Russia with as much as possible so I not only smash the defenses at the border, but also can withstand any Russian counter-attack. I will usually have 4 main thrusts; 3 in Eastern Europe and 1 from Finland. If Russia plays it right, they could counter attack and destroy one or more of these armies and really mess up your invasion.