I probably average out between 3-8 hours a day. Three hours if all tournaments are a bust and around 8 if I make it to a final table. If I make it to a final table, that may be the last one I play that day, because it most likely took a long time. Plus, for me, it feels good to finish on a positive note. If one tournament does take 8 hours or longer I definitely do not play any more for that day, I literally need to do something else, something physical like; go for a walk, bike ride, gardening- usually something outside when the weather is good or if the weather is bad the treadmill or some form of indoor exercise.
A healthy number of hours to play is probably very different for each person on any given day. A healthy number for me would probably be 4 hours on most days with a couple of 8 hour (or a bit longer) days a week. Sometimes I am in the mood to play poker all day, but that’s rare, probably like 1-2 times a month. When I say all day, I mean right when I wake up to when I go to sleep again. Of course taking breaks here and there.
I can play up to 4 tables well, If I do more, it becomes a bit chaotic
And I just play super tight.
As for fatigue - I try not to let myself get to that point and if I do, I take care of it with perhaps even more sleep for a while. I also like to keep a fairly regular schedule; waking up at 8am and going to sleep by midnight, aiming for a minimum of 6 hours. I find exercise and fresh air help me get better quality sleeps. If I feel tired during the day I take a nap. Listen to & take care of your body, it will server you much better.