wow , you must have a lot of customers to be doing only quarterly 
No not really. In fact for the 1st year or so I took a fair amount of financial pain as I canvassed to fill in the BIG gaps. But I just got fed up with customers glaring at me when I turned up. One old man actually said: "Hello Simon did you forget something?". And I realised then that it did seem like yesterday that I last did his house and in fact his windows absolutely did not need cleaning.
So that's why I bit the bullet and pushed all of 'em out to 3 months with just one or 2 exceptions.
Nobody complained and these days they actually welcome me when I turn up because by then they could do with a clean.
Believe it or not I'm a month behind with some accounts :
