As a window cleaner the only way you can function properly is with regular, repeat custom. You need to offer, and insist upon, a regular cleaning schedule, be it monthly, 6 weekly, what ever you decide is best for you.
If people want to ring you as and when their windows are dirty they are classed as "one off's" and should be priced accordingly.
When you canvass, or when someone enquires about having a window cleaner you simply ask them if they're going to be a regular monthly / six weekly, etc clean; the vast majority will say yes as that is what they'll expect anyway.
Good luck
