Horses should be fed at least twice a day (personally, I think at least 3 times in an absoluted minimum). However, the more time they can spend grazing the better. In the wild, they would graze 12 hours or more per day. The more time they can spend grazing, the better it is for them physically (e.g. prevent colic, ulcers) and mentally (prevent stress/boredom related habits such as cribbing or wood chewing).
The only problem is weight control. They should eat a low-calorie but high-fiber diet so the eating time is stretched out, they have the necessary fiber and don't put on weight too fast. If they start putting on weight, one need to restrict food intake. One of my mares puts on weight easy, so I use a moveable fence, starting her on a small area and once she has really eaten that down then enlarging it, but never giving her an unlimited about of ungrazed pasture. However, which solution is best for you depends on your individual circumstances. Perhaps just giving a limited amount of hay a few times per day?
Certainly, one should not worry about a horse getting too much grazing time (except for the weight issue). On the contrary, the only risk is too little.