The time it takes to mow an acre of grass with a lawn mower depends on the size of the mower deck and the speed you're mowing the lawn.
Assuming your lawn mower deck is a 21 inch mower deck and you mow at an average mowing speed of 3 MPH it will take you around 1 hour and 55 minutes.
So just assume 2 hours to mow an acre of lawn with a riding lawn mower depending on size of your deck.
If you have a larger tractor with a brush hog mower then you could most times mow that same acre of lawn within 45 minutes to an hour and maybe less depending on size of mower deck on the brush hog.