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.