I wouldn't leave it on 24/7. Ideally you would have the grazing muzzle on in the day when the grass is at its sweetest and then remove it at night when the sugar levels in the grass are lower. Would you like something like that over your nose day and night every day? Muzzles have their use with laminitic horses and ponies but they do need to be removed after no more than 12 hours IMO.
I would only use a muzzle for a day or night if possible ( I then stable). Even with his sheepkskin lining, the damned thing can rub, and I just prefer to let him have a breather, and get some roughage (hay) into his guts at the same time.
My cob and little welsh live in their muzzles nearly 24/7 as both live on fresh air They get a 20 minute break in the mornings after a bit of breakfast (for supplements), currently are getting an hour off late afternoon as the paddock has been eaten right down, and then another 20 mins or so in the evening. They are about to swap over into a paddock full of grass so the muzzles will stay firmly on with a quick 20 mins after breakfast
You will be amazed at the amount they can still eat with a muzzle on!