Interesting topic. There is a business term that internet and energy providers use- churn rate- and you are supposed to have a low one, and it is one of the indicators used to asses a business and possibly sack the MD.
Mine is a lot higher than it should be based on a base of over a hundred. Many are my fault but this is coming down. Also a lot of my custys are other window cleaners rejects so I do have people complaing before I have even done the job.
Just as frustrating, you do a good job, they believe there windows no longer get dirty and they go to a longer period between clean, which again translates into lost work.
I think Trevors figure 4.5% is a good one and probably about as good as we can expect.