First of all every horse needs more or less attention, so my first answer is all depends on what your horse needs and also the time you can spend with him. After a typical period spent with my horse, I take care that the time is not too long compared to the time I stay (I come 2 hours I will not work more than an hour) in order to be able to spend moments of pleasure without work.
Moreover, the psychology of the horse must be taken into account in order to respect its own rhythm, its abilities and adapt to him. I vary enormously the sessions, jumping, dressage, walk, games, .... I also vary the difficulty of the sessions and exercises that I ask. For example, if my horse succeeds in an exercise while him is usually a disaster, I congratulate him, I do an "easy for him" exercise and then I bring him back, even if it's less than the time I have planned. The fact of doing a "simple" exercise (that the horse succeeds well) after a complex exercise, shows that after the difficulty there is simplicity. But also that it is not because it does one thing well that one must stop everything. By doing an exercise that he likes to achieve, it is as if it were a reward!
I think it is a mark of respect when we accept what the horse offers us. By respecting what a horse gives us, the horse will soon realize that the more he succeeds in doing exercises, the more he will be quiet.
I hope this helps, bye :)