You can't choose how many minutes a player gets in general. However, you can estimate how many minutes a player will get per game. Basically, a starter gets between 30 and 40 minutes, and a backup 8-18 minutes accordingly. If you want to get a trainee 48 full minutes, either play him in two games a week as a starter, or make sure you have only 9 (for single-position training) or 8 (for double-position training) players in your active roster. Put your trainee not only as a starter, but also as backup and reserve at his training position. If he doesn't get injured or fouled out, he'll get his 48 minutes.
If you want to train PFs, you have to play them on the C position for most training purposes.
This guy should get Inside defense and rebounding for now. Try to get him to triple proficient at the end of the season in ID, IS and Rebounding. Then you can either go on pushing his inside skills, or switch to training forwards in Jump Shot and 1 on 1.