So, everyone who isn't under a rock has heard of the saying "I practiced until my arms fell off" and a lot of musicians take this to heart, practicing so much until they can't even play anymore. I am here to say please don't do that. It doesn't help you at all. You lips, your fingers, your hands, they are all like muscles. If you work out your muscles to much, what happens? They actually get weaker. And if you are wanting to get better, that definitely is not going to help you at all, is it? No. The reason to practice is to make yourself better, so here is the basics. You need to practice until you get CLOSE to your breaking point, not over it. Once you feel yourself straining a bit to play, stop and warm down. Don't worry, if you do this, you will get better and eventually be able to play even longer. But if you continually play longer than you should, you will not get anywhere. So, in conclusion, practice until right BEFORE your arms fall off, figuratively speaking.