I was watching a documentary entitled "Imagine Being a Concert Pianist" . They featured a bit on how Julliard trains its students and one of the professors, Veda Kaplinsky, mentioned that a good 4-5 hours would be sufficient, and 6 is stretching it. Apparently, just like exercise, when one overdoes it, it compromises muscles and blood flow. My teachers were quite varied - my older professors would say 6 hours max, and my younger professors would go for minimum of 3 hours and max of 4. If one is busy, at least play for 30minutes just to continuously build muscle.I do think that one needs to determine one's own arm, finger, and hand stamina. When tired, it may be wise to simply stop and rest for a few hours.