Unless your pup is sleeping (rest is really important!), doing little 15-20 minute training sessions every few hours is a good start!
If/once you join a group class as your pup gets a little older, you will see they'll be able to handle longer training periods but the most important thing is to pay attention to how they're feeling every second.
You want to make sure that they're in the right mood and not over tired, for best results and ensure training is something your pup always enjoys!