Tosh, I stated offering 8 weekly cleans about 3 years ago. I did it one estate at a time, and currently have 3 estates on 8 weekly cleans at +50%.
I explained to the customers that I was far cheaper than many of my competitors.
In order to make my business economically viable, I had two choices. Put up the price substantially, or pass the under priced jobs to another cleaner.
I told them that I would be prepared to carry on doing their windows, but only on an 8 weekly cycle for 50% extra.
I pointed out that they would save on their annual window cleaning costs, and that it would then be economically viable for me to continue doing them.
All but one accepted this, I was a bit surprised as I had expected to lose more.
As has been said above, I too find very little difference when I clean 8 week accounts, as opposed to 4 weekly ones. Dai