Don't ever practice any section for more than 15 minutes. Yes, you will get fatigued, start making mistakes, and then go backward. It is counterproductive.
Better to take a short break and then work on something else. You will feel refreshed, and going over different material is more effective than hammering stuff you're already tired of.
You can practice as long as you want, as long as you take breaks and work on different things.
A lot of this is choosing the passage you're going to work on. If it's too long, an hour of practice won't be enough, if it's just right, then 10-15 is enough. You will have to come back to it the next few days.
Note, some particularly athletic passages/exercises are more strenuous and you should be more cautious about. Running 16th note scales at 144+ for 45 minutes is going to mess up your hands.