With some it is quite litteraly warming up the muscles and how warm depends what level of work they will be doing. A walk trot dressage test requires less warming up then a grand prix does.
Other horses it is a case of working them in, getting them to think, settle, concentrate and listen. That can take quite a while depending on the pony.
Stan in a good mood and for showing could come off the wagon, do a walk trot and canter on each rein (so 10 min warm up,) and go in the ring and win and go champion. I only did this when running seriously late.
Stan in a quirky mood could take over an hour to get him to stop jumping his own shadow and to listen to me.
Stan doing serious dressage took 40 mins to make sure he was listening, quick off the leg, supple, bending, working trough correctly and that I had all my transitions within gaits (collected, working, medium and extended) spot on.
Danny you had to do alot of walk to warm him up because he had a side bone and if you did alot of troting he went hopping lame.
Rolo you had to essentialy school him into a stupor so that he didnt get too excited in the show ring and didnt take hold.
Pride it was a case of alot of transitions and a good forward gallop to get him thinking forwards and moving forwards (if he didnt get his gallop he often got stuck in reverse).
Max (high level dressage horse) also needed a good forwards ambling canter to open up his pipes and get him going forwards off the leg, then we normaly settled into lots and lots of bending, then after that it was a lot of latteral work, then just before going in the ring I'd give him anouther good canter, stick a flying change in or two just to keep him thinking and we were good to go. His transitions were spot on when you got him going forwards so didnt tend to do many.