you have a regular clean to keep them clean at a "maintained" level. If you didn't keep going they would be really stinking.
Up to the customer if they want a regular maintained window cleaning service, or a as and when service. You charge then as they want the windows to be cleaned.
In the uk, we have an approx 4 weekly clean as standard.
In the US they have a yearly clean as standard.. obviously going to be a hell of a lot dirtier, and a more expensive clean..
Now explain that to your customers, if you see where im coming from.
I have set my prices that way, i say i charge "x" amount for a regular 4 weekly clean, i just turn up and do it, or they can have an appointed clean at obviously a higher price and charge "y" amount. I tell them why the difference in price, and allow them to choose what they feel will suit them.
I dont force them to use either one but if they choose the 4 weekly then start messing about i will advise that the price charge was for a 4 weekly, and now they have changed the clean regularity to an as and when this will be the new price.. I explained my reasons to them in the first instance so they knew what my charges would be..
Im honest to my customers i explain why they have problems in finding a regular WC, as its most likely they keep saying not this time, not this times when agreeing a regular monthly clean price, and the window cleaner decides to drop them..
if they dont like that, then they are open to find another window cleaner.
I feel honesty is the best policy.