hey there
you get "maximum" training after 48 mins.
Anything over 48mins will not improve the training, since it's already optimal.
So minutes over 48mins is just to "tune" the gameshape. Try to get your players between 50-65 minutes and keep it steady. If your player plays 50 minutes, he not only get the best training (100%), but also has a good chance to have a very good game shape. Whereas if your player plays 90 mins, he also gets 100% training, but he ll most likely take a hit on his gameshape.
Besides, you need to give your trainee's the minutes.. so anything over 50 minutes is a waste in my opinion (for the positions you're training)
Hope i could help, otherwise ask again :)