Years ago when I first started, it was generally accepted at every two weeks. but when my round increased to too much to do, I increased my prices and, at the same time, put them all to 3 weeks, saving them money. later, when I moved to another town, all the window cleaning had moved to 'monthly', so I went along with that. Now my work has increased again, and for the past three years or so I have been working at a strict 5 weeks. Although I do others at 6, 8, 10 and 13 weeks (big jobs), I think at 5 weeks they're starting to look dirty. Afterall, it's maintainence we are offering and not on offs. If anyone says anything to me about it being that time again or it always rains now that you've cleaned them, I just let it pass.