It depends on how much grass/hay she can get through the hole in the muzzle, if she learns to eat through the hole. My donkeys did, in fact they learnt how to enlarge the hole, so I'm constantly having to make it smaller again. With my donkeys (that came to me obese), after two years of using muzzles I have a feeling they need it on most of the time, especially when pasture is lush. In practice I lock them into a "slimming yard" at night with a little roughage, like acacia branches, and let them free-range the pasture with the muzzle on during the day, with an occasional "muzzle holiday", maybe one day a week or fortnight, or an hour during the day, when the pasture is dry. Just monitor the horse's condition, and experiment. It's important not to over-restrict especially at first, but to make changes gradually, like with all feeding.