Hi Wayne,
Yeah I do tend to find that to be honest. When I canvass I always go in at 8 weeks now, that way I can almost garauntee that they will want there windows cleaned every time I turn up!
Some bigger places do go for 12 weekly, I have a few as you can see... But in the main all my houses are 8 weekly. It's a bit more leg-work when you first setup and go out canvassing, but well worth it in the end.
Andy