You should NEVER play so much that you experience fatigue and/or pain. Believe me - been there, done that, I paid dearly for it.

Otherwise I think this should be an individual choice. My personal opinion is that you should never push yourself in that extent that you start to get negative feelings around your piano playing. There is no point in getting exhausted. I know this attitude might upset those who are fostered in the "no pain, no gain" spirit, with old fashioned teachers who drilled their little students to tears and got "fantastic" results - there are plenty of stories from big stars about this kind of drill, the idea that you must experience blood, sweat and tears in order to "get somewhere". In some places it is a whole philosophy, not to mention a moral thing - if you push yourself you are a "better" person than those who are just having fun, according to this moral.
I mean, it could be right. For some people it could be right. Some people are masochists, deep down they enjoy being tormented. But this approach also has got its victims during the years, some students have been totally broken down by it. Some of the big stars lived unhappy despite their apparent successes. I have made up my mind that it is just rubbish. Pianists who truly enjoy every moment at the piano are the best pianists in the long run - if you ask me. And they are also happier as persons, and what could be more important than that???
So, IMO you should follow your heart and do what feels best. If you don't feel like playing, then don't play. If you feel like play, then for God's sake, PLAY. At least until you start to get tired, like already mentioned. But if your body sets some limits, you can always go on practicing away from the piano. You can memorize, analyse, listen to your own recordings, plan your upcoming exercises and so on. Or go out and do some workout and mental exercises in order to stay strong and fit.
It is also true that some people can practice for 20 minutes and do it very, very efficiently and smart, and others can practicie for 8 hours and achieve practically nothing.
So there is no absolute answer to your question, this is an individual thing. Generally I would recommend far less than 8 hours a day, but I know those who play even more than that and are comfortably with it.