Around here we do and guess what all my round is built on fortnightlys.
I have 6 monthly rounds but they are all new build site the rest are fortnightly.
Its an advantage of living in the north of england
You know, once you get too much work, speak to your 'fornightlies' and suggest you increase the price, but decrease the frequency to four-weekly.
I do similar, but instead of having the more common monthly frequency, for number of years now I have priced them very well and suggested every-other-month (six times per year), and they mostly go for it.
I make a point of saying that many window cleaners still rigidly stick to four-weekly cleans, yet in my opinion, they don't need doing so often, so they actually save money, and still have a regular - fully insured - window cleaner.
I have a lot of work like this, and it works really well; they even seem pleased to see me.