They are very low (motor) power, so they need to run often, in order for the grass not to grow too tall. Here in Denmark they usually run at least three times

They are very low (motor) power, so they need to run often, in order for the grass not to grow too tall. Here in Denmark they usually run at least three times a week, but very often every day of the week. How many hours they run each time depends mostly on the ratio of the lawn-size to the mower-width.