I just charge the same no matter what frequency they want. All I do each week is print a weekly worksheet off on George and then go and do whats on the list. It makes no difference to me how often their name comes up.
Six to eight weeks is becoming the normal interval. Better for business but don't say you are going to charge them extra, just charge a good price for all your jobs.
It gives you an advantage over the other window cleaners if you are flexible. I had a new enquiry last week from a woman who wanted every six months! Yeah, no problem, load it on the computer, its all money.