I once went to a window cleaning marketing learning day at Ionics. Whatever else anyone may think of the company, I reckon that they are pretty decent at marketing - particularly to newer window cleaners.
One of the guys attending put it out there that the way forward might be window cleaning on demand, but charging about 3 the normal rate. This plan was dismissed, including by myself. However, afterward I thought that maybe it could work under certain conditions, the main ones being that a very high rate is charged, rather than being "within x days", it is carried out next time working that area and possibly the main one, that only a very small percentage of a cleaner's work is of that type.
Personally, I think it's too much hassle.