Well... First it's not implemented
Second: for now it's not for sure, it ever will implemented.
Third: the seventh trainee... in theory he'd lose just 8 minutes. But: it's impossible to plan. Stamina, how good is the Backup, wenn is a call (out of bounds, foul)to substitude etc.
It'd be just more easy to train the six (or 3) guys.
I unverstand the idea, but for me, i dort like it. Training has to be hard.
I also hate it, if the trainee Fouls out in Minute 47or48, but that's Part of the game. Train stamina in two-game weeks or some defense before pushing driving to lvl20+
the players are already much better than some seasons ago. Training works how it is, but you have to sacrifice a bit.
Manchmal ist man der Hund - manchmal der Baum.