I firmly believe that anything more than four hours is too much. Practicing is about correcting bad habits. Maybe you are lifting your fingers too high off the keys, or you keeping missing a certain fingering, etc. These are all bad habits that your mind follows because you made these mistakes over and over again when you were young. To fix these bad habits you have to teach your mind good habits by repeating the motion correctly over and over again until it comes naturally every time.
Bad habits are very hard to break.
It may have taken you years to start doing something the wrong way automatically, and it will take alomost as long to do it the right way automatically. But the key is to do it a little at a time, day by day. Your mind can only let go of a bad habit a little at a time, so practicing more than four hours doesn't do too much for you because your mind can only let go of an instinct a little at a time. Next you need a suitable place to practice. Many musicians try to sneak into the school auditorium or recital hall to practice because the sound in these rooms is rich and wonderful.