Been going nearly 25 years, when I started most people wanted their windows doing every fortnight, and of course there were the odd ones that wanted once a month.
Many years back I changed all of our work over to monthly, and guess what, there are some that want it every other month. No matter what you choose there will always be someone who wants to be different.
I decided that, I would only take on customers who wanted a monthly clean, and 9 times out of 10 when you tell them that, they agree, the ones that don't, I don't take on.
You must run your round, don't allow your round to run you.
If we continue to have window cleaners that will take on cleans at 2 monthly, and 3 monthly intervals, you will end up with the American situation, where they will ring you when they want you.
Stick to your guns, and educate your customers that window cleaning is not about cleaning dirty windows, but maintaining clean ones. Then look for the add-on services that you can provide to these regular customers.
If people don't want their windows cleaning at the same time as everyone else on your round, then they are looking for a different service. that is window cleaning by appointment, in which case the price should be completely different to the price you charge your regular customers. At least double your normal price, and probably more than that.
Blessings,
Graeme