It really depends on the horse, if it is a really agitated horse you should work more, for example my mare is very agitated so I work her for 50 minutes, so I trot her for 25 min and gallop for 25 min with breaks between them of course. It really depends on the horse, if it is a calmer horse I wouldn’t work him so much. Also when you are competing the next day you shouldn’t work your horse so much, I work only 30 min mine when there is a competition the next day. It depends on many factors
I would say walking-energetic/active walk - for about 10 mins then trotting excercises for 15 mins and then I usually start cantering for a few mins and do a few excercises and trot and canter.....after you do this do different excercises that make the horse use their brain and think.
It really depends on the horse. If the horse gets tired easily you might want to build up how much you work them slowly. If the horse is fit and is exercised regularly then its okay to ride them a bit more. Obviously you DON'T want to start galloping the minute you get in the arena. The amount you exercise is up to the rider and the condition of the horse. I usually walk for 5 min, trot for 10 min, then canter for a bit go over jumps and stuff like that.