I do a quick review of what the horse already knows. Ground work first, usually things like yeilding the HQ, backing, yeilding the FQ then I do some sending and circle on the long line, with turns, stops and changes of speed. I can see how he moves, what kind of mood he is in and check for any lameness.
If all is well, I mount up and start off slow, walking, stopping, backing, a little more speed, such as a fast walk or slow trot/gait, and several changes of direction. If the horse knows sidepassing, yeilding FQ or HQ under saddle, I do a bit of that. In general it is just a quick review of things the horse already knows.
After we work through that, I move on to the newer stuff I am working on with that horse. After lessons we, just do a nice relaxed walk out. Then I tie the horse up for about 30 minutes and let him think about his lesson before I unsaddle him. I am aiming for a calm relaxed, happy horse that enjoys lesson time. Not one that is anxious, frightened or angry.
I have no time limits. Some days when things are going well we have a longer lesson and on other days, when things are not so good such as when wind is howling, the neighbors dog barking, the tractor is cleaning out the barn, etc. I can tell my horse is having a hard time focusing on the lesson and I make the lesson easier and shorter.