Everyone who plays minutes at the position you decide to train will get some training. 48 minutes is full training. Anything less than 48 minutes is not ideal. Anything less than 48 minutes is an inefficient way of training. For example, if your PF played 35 minutes at PG, he would get significantly less training than if he had played the full 48 minutes (so I hear, and so I have experienced to an extent).
If it's in your power, get the full 48 minutes or don't get any at all. Definitely shoot for 48 minutes with your trainees.