Players don't need to get over 48 minutes to get training its just when they get 48+ minutes they get full training. Any player who get some minutes in the position being trained will get some training. You might not think they get training because a skill hasn't 'popped' however skills have sublevels which arn't visible. Stamina doesn't drop due to minutes, what your thinking of is Game Shape. There is a random element to game shape but ideally you want to be giving players around 55-70 minutes game time a week to give them the best chance of strong/proficient game shape. Stamina on a player may drop once or twice randomly during the season with many people using the all star break or end of season to train game shape/free throws.
Also with training the effectiveness of training depends on the number of positions being trained rather then the number of players being trained. For example 6 players get 48 minutes at PG/SG, 5 players get 48 minutes ar PG/SG/SF. The players trained at PG/SG will get more training because less positions are being trained. When your fairly new 2 position training i think is the best because it allows you to add depth to your team, one position training is better if u have a solid team that needs a couple of stars or if you want to develop a NT player.
The effectiveness of training also depends on your trainer and age of your players. Young players train faster so ideally you want to get a 18 or 19 year old and train them up. A level 4 trainer is regarded as a good starting level until you have more money coming in then you can see about getting a higher level.