Doing gridwork I start working up a line of trotting poles after about 10 mins of warming up, then will jump for up to 10 mins, have a breather, jump, breather and on it goes
Completely depends on the horse and what they/your doing!
Youngsters start jumping much quicker but have much less intense sessions and there generally alot shorter.
Older ones have more warming up on flat then more intense jumping stints/going for longer
Mine lessons are generally half an hour or so long. Have about 10 mins warming up first (generally not included in the half hour lesson). Then depending on what we are doing a typical lesson could be, canter poles, small grid, bigger grid, bigger grid.
Then going round a course incorporating the grid or parts of it. Then depending on progress etc going round the course again with all the jumps up. Depends on how the horse is going, and how the ground is.
weekly, and we start almost straight away BUT i hack(2.5miles) to lesson so horse is warmed up. Have a canter on each rein and away we go. Lesson is 1/2hour long and will vary from grids, to courses or to other exercises (instructor asks what i'm up to next, so therefore tailors lesson). Then hack home so horse cools down nicely
15 mins warm up, which i try to get done before the actual time of the lesson starting. then about 20 mins if it's gridwork, or about 40 mins or more if it's exercises and/or courses, but this is broken down into "do an exercise, have a chat about it, try to do it differently, do it again, analyse it again" etc, so lots of breathers for the horse in between bouts of exertion.