Good custies will be reasonable and appreciate that dirt starts to accumulate the minute you finish cleaning them. They may joke with you about it form time to time but on the whole the're OK with it. The ones that kick up a stink about it I try to educate. If that doesn't work I don't want the hassle. I take them off my books.
I'm also reasonable in that if I know the windows are going to get filthy almost immediately, depending on the reason why, I'll adjust my schedule. (ie: If the neighbours are having building work or cavity wall insulation installed.)
If your custies see you being reasonable when it's called for they are more likely to be reasonable with you when the weather is unsettled.
A few weeks back I'd just finished the front of a property when the Cavity wall guys turned up. I was still in the area at the end of the day so, uninvited I went back and re-did the fronts before I went home. The couple didn't expect me back but were very happy that I did.
Keep em sweet! (Within reason of course)