I normally allow two cleans but will stretch to three if the amount is small. I only write a reminder letter if it's gone past two cleans. If no-one is in when I visit for the trhird clean, I normally don't clean and write a reminder letter. I politely make it clear that, like all businesses, I depend upon cashflow for survival and ask that future payments are made more promptly. If they are persistent, I drop the job.