Good afternoon, sometimes when I have been playing cash for more than two or 3 hours I feel tired and I think I don't play at the same level. What do you think is the reasonable time to play poker online without losing quality of play?
Good question. The isn't a specific answer as it all depends on the person, but there are some things you can do to try to improve your endurance.
1) Be properly rested. There is no point expecting to be able to sit down for an 8 hour cash session at 9am on a Saturday if you've been out until 4am clubbing. You wouldn't do that if you had work, so why would poker be any different?
2) Diet. Eating the right food is really important for maintaining focus. Also, make sure you drink plenty to stay hydrated, water is great; don't drink too much caffeine as eventually you will crash.
3) Exercise. Being in good shape helps reduce fatigue and keep concentration levels high. A short 20-30 minute workout in the morning will do you wonders.
4) Get your house in order. Sometimes when I play, I know there is other stuff that I need to get done. It could be the laundry, or vacuuming or building the shed that your wife has been asking you to do for months. I'm not saying that poker should be your lowest priority, but if you have a list of chores to do that is nagging at the back of your mind, you will not be able to focus 100% on your game.
I'm sure there are other things that people can suggest as well, but I think this is a good start.