You need to use it ASAP to swarm the German player. The longer you wait, the more irrelevant infantry becomes in the game. Ideally its worth the munitions cost, since manpower is much more precious than munitions. Its one of the reasons why US strafe rapes so hard. You can probably activate it in early-mid game, when the German player only has 4-5 infantry squads on the field.
Combine your new squads with your current ones to totally outnumber and swarm the German player. Wehrmacht T2 is especially vulnerable to this, because Grens must spend over 300 manpower on vet for them to be effective, letting you outnumber them even more.