You cannot precisely instruct your coach to give a certain amount of minutes to your players, but you can take some measures to make sure they stay in a certain range of numbers. For example, you could set a trainee as Starter, backup and reserve, and then make sure to only have 9 players active for the game. If you have 4 other starters and backups on the other positions, your coach won't sub your trianee, thus giving him 48 minutes (barring fouling out or injury).