[Paul Coleman]

There are no rights and wrongs, some people work in areas where customers prefer 6 and 8 week
intervals.
Some work in areas where customers would rather have them cleaned monthly.
Some window cleaners prefer longer intervals because it allows then to do extras like fascia, conservatory roofs
and solar panel cleans.
Just work to what suits you and your customer base.

Exactly Sean.  I have run more than one system over the years.  I have adapted it to a number of variables such as my workload at the time, the general economy, computerisation (meaning that simplicity wasn't quite so essential any more), my finances and occasionally, my varying motivation levels.  I also had to work around a health problem some years ago.  So, as you say, do what you need to do at the time.