My pea tiny farm produces enough grass so that fall and spring the horses graze 24/7 and only the oldie gets hay (his alfalfa as he’s not an easy keeper). In summer the horses stay in the barn in the worst of the heat so I feed them hay during those hours. In winter, they graze during most of daylight hours and I feed hay overnight.
I do plant winter grazing, annual rye grass. I do maintain my pastures but I wouldn’t call it a heroic effort. Mowing, lime, fertilizer, rotation and occasional irrigation. The winter grazing reduces weeds a good bit. I have approximately one acre (maybe a bit more) in grass turnout split into two paddocks and a third “dry lot” paddock that grows some grass in summer. Two horses. It sounds crazy, but it’s not an uncommon stocking rate in my area (Pensacola).
IME a lot will be determined by the soil on the property. Too sandy and it won’t hold water or nutrition and your grass will be poor. Too much clay and the ground will hold water too much and the horses will tear it up too easily resulting in not enough grass. Stocking rates will play into that too. The less ideal your ground is the more acreage you’ll need to grow the same tonnage of grass.
New pastures planted on freshly cleared land will do best if given a whole year to grow without horses. I don’t know first hand how much better that is as everyone I know that’s done that has started grazing after the first mowing. 
Barns need to be as open air as possible. Lots of fans too.
YMMV