It really depends on a lot of factors: will the horse be on pasture? If on pasture, good pasture or overgrazed pasture? How big is the pasture, with how many other horses? Is the horse a hard keeper or an easy keeper? Etc, etc, etc.
My mare, an easy keeper, in a 1.5 acre pasture - alone, needed about 40lbs of grass hay a week (I fed it free choice but she usually ate 40lbs or less/week) and 8-10lbs of alfalfa/day during the winter. During the summer, she does just fine on pasture grass.
So, in the deepest winter with very little pasture grass to be had, she'd eat about 2/3rds a 60lb bale of grass hay and about half an 160lb bale of alfalfa.
She recently moved to a different, larger 5+ acres, pasture so I may not end up needing to feed her much hay at all this winter. Time will tell.
It depends a lot on the horse for how much they need to eat. You can estimate that they'll need to eat about 1-2% of their total body weight in hay per day. 1% for easier keepers, and 2%+ for hard keepers.
ETA- Oops! So many other people saying the same things!