If I was you forget about changing the cleaning cycle, if they have other companies who are cheaper and will still clean their window on the cleaning cycle you are already doing it at, why would they want to have their windows cleaned less often but still be paying more than the other cleaner.
Have a look at the job and price, if its is still profitable at the new lower rate then go with it, you could also try offering them a price cut for say 12 or 18 months, after this time you will re-alter the price back to its original figure.