I do mine when i just about cant see though them...which is about once a year.
I too agree, cant beleive people are so funny bout getting their windows done regulary, i have a lady that rings me EVERY month to see when im coming and to arrange a good time to come.
If i had a window cleaner and he was late or whatever...i wouldnt go to the hassle of ringing them! At least not after only 4 - 5 weeks lol.
As said before they need to get out more. And i agree, wfp you could go 3 months really easily. In fact i have a few 3 month'ers...i put my round to 5 - 6 weekly, so the every others are 10 - 12. The 10 - 12 are noticably worse than the others!