I live on 10 acres of nothing but grass-grass-grass. It's a blessing and a curse sometimes. I would love nothing better than to leave all of my horses out ALL of the time, but no can do with minis. Right now even the non-show horses have to be brought in for half the day just to keep them from getting too heavy.
But your question was about show horses. Between shows, they are allowed out for 6-8 hours per day. I start restricting them about 2 weeks before a show such that the week of the show they are out for only 1-2 hours. They are, of course, worked during this time on a regular schedule, round pen, golf cart, or driving.
I find this works for most of my horses. I've been blessed with "easy keeping" show horses, though, who can go out on grass and not get bellies. Maybe it's just a combination of exercise and the way I feed, or it's just their natural inclination to stay in shape ??? I do like my show horses to lead as natural a life as possible, so I'm happy to say that mine enjoy liberal turn-out.
Robin C