I usually get this question a lot, but the truth is there is no right answer. It really depends on two things. First, the house, how often do the windows get dirty? If the owner has pets or small children, if the windows are in an area that gets hit by a lot of wind and rain then they probably get dirty quite often. Second, the home owner. Some people can’t stand dirty windows, not even a little. I would say at least once a year you should have your windows cleaned. It's all up to you though!