I like to charge as high as possible, and keep the frequency to 2 monthly. I usually explain that most window cleaners prefer to do them every four weeks, but say that I don't think they need doing that regular; and then quote a high price for every other month. I've got loads on that frequency now, all well paying jobs, and they usually appear happy to see me when I turn up, rather than 'Oh no, not you again' type attitude.