The time taken to mow an acre with a riding mower will depend on how powerful your mower is and the length and condition of the grass before cutting. If you have a mower designed for medium-sized lawns (which are, on average, around 1 acre at the most) you can expect the activity to take around half an hour to an hour, meaning you will only cut for around 11 to 22 hours of cutting time annually if you mow your lawn twice a week during the summer months. This is only an average time, though, and this could change depending on your speed, if your lawn features uneven terrain or rough terrain, and whether there are any obstacles that hinder simple mowing possibilities.