Have three horses; one easy keeper, one medium, one medium+. They are 14.1H, 14.3H and 15H, what I consider average size.
In winter, I give them 2 flakes of hay each morning and night. The rest of the year they get 2 flakes of hay at night only, none in the morning.
But this year, have had so much rain the pasture is in great shape and I haven't fed any hay since March
They are also fed a low starch feed, Safe Choice by Nutrena. The two older ones get the senior feed and the younger gets maintenance feed with 1/3 of the senior for flavor.
So the easy keeper gets 3 generous cups (measuring cups) of feed twice a day. The medium keeper gets 4 generous cups twice a day. The oldest horse, a medium+ keeper, gets 5 generous cups twice a day.
They are on 3-4 acres. Have had four horses here, and usually have to feed hay all year. The three of mine together I use about 1/2 bale per day. So I think 1/3 of a bale per horse per day is a fair estimate, depending of course on the size of the bale!
Had an appendix QH gelding with the digestion of a TB. He ate 1/2 bale per day, plus added alfalfa pellets, and honestly could have used more. He ate as much as my other three combined.
I avoid round bales as much as possible because I have one easy keeper, but used them last year when I had them off the property. I bought high quality bales and put them under a carport so they stayed dry. Still the horses ate that hay extremely fast.