Some consideration has to be taken as to what sort of ground your 40 acres is. Is it one big square, flat field? Or is it a series of smaller fields, hills, creekbeds, trees, ect?
I mow 45 acres at my place. I have a commercial mowing business. I own three 15' batwings and three tractors big enough to handle them. They would be totally useless on my 45 acres. (Hills, several creek beds, small fields, woods, ect) In fact, MOST of what I mow, even my 7' mower is just too big and too cumbersome.
I mow the bulk of the place with a 6' mower on a 47 hp 2wd tractor. I mow a few acres at a time. I'll spend 2 or 3 hours one afternoon, maybe an hour or so a few days later, just whenever I have a few minutes to kill, I'll mow a while until it's done. Never gets to be a burden, always stays mowed, and I get plenty of "tractor therapy time".
Needs are relative to conditions.