Well, I will have about 4 acres of grass. But I'm buying a tractor with a 6-foot-diameter mower from my neighbor. He says it mows at the rate of about an acre every 10 minutes.
Hopefully, the micro-robotics industry will soon live up to its promise of molecular-sized robots which will climb each blade of grass, and cut them off at 2 inches
(Without doing the same to people, or anything else standing on the lawn.)