I figure an average of 20 lbs of hay per day for my horse (~1000 lbs, easy keeper), comes out to about 4 tons per year. My horse has pretty limited access to pasture, so gets hay year round. At that rate, 1 ton would last 3 months.
Of course, it doesn't get very cold where I am and he's stalled, too (but not blanketed). If it gets really cold where you are and your horse is outside 24/7, you'd likely be using more than 20 lbs of hay per day.
I have any guess on the acre of grass, but if it's heavily wooded then I'd guess you'll need to be supplementing with hay.