If he plays 23 minutes at PF, and 25 minutes at C, he won't reaped the benefits of training because he didn't receive 48 minutes at C, right?
Correct, he will only receive 25mins worth of RB training for that week. It does not equate to 25/48 of maximum training for that week though, it will be a bit less than that.
Rebound training is always for PF/C or even more players. So his player will receive full training.
Your answer is correct, if he's training e.g. IS for C only.