No matter how many minutes your player play, it don't effect the stamina.
If you want to get a batter stamina for your players, you have a "Team Training" called "Stamina".
Usually a player need around 2 weeks to get an up on stamina like on FT. A coach lvl don't effect stamina or FT training (so the same training speed is with coach lvl 1 and lvl 7).
Cheers!
This is where we hold them!