You can still play excellent tennis on 90-120 minutes of training per day. This training will need to be intensive ground strokes and net play. The serve will be worked on less and playing sets and practice matches are virtually out of the practice schedule due to the breaks in the play being less intensive than training. If you are already a very strong player, you can keep your strong with this amount of training. When a player is trying to make huge progress more time must be spent on the court. Developing the game is very time consuming and labor intensive. Plus, the more you play the more you have to take care of your body off the court.
In response to training recommendations for ages 5-15 years I think a 5 or 6 year old should play 2 or 3 times per week for 60 to 75 minutes, and should be playing several sports to develop overall athleticism. Ages 7-9 can play 3 times per week for 90 minutes plus play 2 or 3 other sports including soccer, golf, baseball, basketball, martial arts. Ten-twelve year old's should train 4-5 days per week for 2-3 hours depending on the type of work-outs. At 13 and up the player can train 15 hours and up depending on overall physical development, and this includes off-court injury prevention work for body balance, flexibility as well as strength and speed.
This training I would recommend for a player that wants to be world-class someday.