Well personally i only play a player a full game if:
a. he is a trainee and i need him to get his training minutes
b. its a scrimmage
My players are pretty even though, if u did have a star player who is a lot better then everyone else on your team u would probably give him 48 minutes in a must win game. So playing the full game their performance will drop off its just a matter of if even with reduced performance they will be better then the backup.