Your customers are paying you to keep the windows clean. Not to turn up when they're dirty and make them clean again.
Exactly. Debbie (Texas Girl) cleans house windows only once a year but charges something like £6.00 per window.
Frequency of clean affects the price. If it's weekly, it's going to be cheaper than if it's quarterly; as there's less grime to remove.
You're paid to provide an honest service, at your agreed regularity. There's no need to think you're a 'fraud'.
One common complaint from customers is that, 'My old window cleaner was supposed to come every month, but he only came when he felt like it'.
Obviously they were on someone's 'B List' (David's phrase), so just stick to the time-schedule you originally agreed with your customers.