48+ is the maximum amount of training a player can receive. If they have a 48+ it means that wont be able to receive any more training for that week, even if you want him to play more minutes.
Now if it has 48 mins for training, that means he hasnt received full training, and missed out on a minute or less. So make sure you can get 48+ for each of your trainees, which just means playing 1 full game without being subbed per week