As I'm on a week-long work-free practice jag, I read these posts with interest. In college, as a music major, I practiced for several years eight hours a day, breaking it up into two hour sections: once early, once mid morning between lectures, once late afternoon, and once late at night. I made sure to exercise hard running or swimming for an hour every day, too.
30 years later I've tried to duplicate this, but find my mind gives out before my hands. I try to combat this by working on different pieces during different sessions on different days, by playing chamber music one day, solo piano the next, etc. Sometimes I do memory work, other times I learn new material, other times I play chamber music.
Ergonomically, carpal tunnel surgery helped my hand fatigue immeasurably. CTS is very common in musicians; something to think about. These days I make sure to stand up and stretch between each movement or piece. Prolonged sitting is murder for your back. And I still work in daily exercise and frequent orange juice breaks.
Bottom line: one can practice for hours a day and practice well, but one must be clever about how to do it.