I don’t have an exact time but I start off with letting him walk around with his head down and stretching out his back then 2 rounds of trot with his head down and 1 round up and on verticle and then 3 canter rounds. Hope this helps!
If the horse is in good shape and the weather isn’t cold, I typically do about one full loop around the arena on each leg and then trot for 10-15 minutes before cantering. If it’s colder or the horse isn’t in shape, extra time should be given to walk and trot so they are warmed up prop
I spend around 15 mins of lose rein walk and trot. Then I shorten my reins and ask them to collect, lengthen etc.. but only in trot. Determine if your horse is tighter or stiffer to one side and do some circles on the side if they are. Finally you can canter and start to do your exercise.
Hope this helps!