6 hours a day is plenty for a professional working musician, since they’re going to be performing, dealing with students etc. Serious students will do up to 10 hours a day tho.
I would hope they are spending many of those hours reading instead of doing the physical exercise.
In my experience playing, after only a few hours, I stop getting any better. I need sleep to solidify my understanding. Although I'm a 15 year amateur music player.