How long is a piece of string?
I try to work my gelding 5-6 days/week, generally one of those days is work in long reins or on the lunge. He is competing Novice Dressage and training Medium (similar to US 3rd/4th level I believe). After a couple of days in the paddock, he will take a good week of solid work to get back to a work mindset, he's a rather reactive little fellow so benefits greatly from regular work.
Each session is generally between 30-60minutes, depending on how he is working.