If you have a dozen window cleaners in an average size town that are all using WFP I would say that the average usage of water would be about 350 litres, take waste into account while producing the water and we will each be using ...say...a 1000 litres a day tops.
during the summer, if you have...what?...between four and five thousand homes?
Well if a dozen of those homes decide to put the sprinker on their gardens or lawns for 5 or 6 hours, each one is going to use somewhere in the region of a thousand litres of water an hour
If I stick the hosepipe into my thousand litre tank it'll be filled up in way less than an hour!
If a single householder left his sprinkler on overnight, he would use more water in 12 hours than all the window cleaners in his town would use put together.
WE would use perhaps that 1000 litres...compare that to factories that use several million litres per day, what the window cleaner will use is a totally piddling amount of water by any comparison.
hosepipe bans on households makes sense, if you go out and clean your car with the hosepipe you'll use a few hundred litres of water in the 15 minutes or so it'll take, if you have several hundred people doing the same thing it adds up to a lot of water, same with with having a several hundred watering their gardens.
So the savings are considerable, but yet again, what we use is a drop in the ocean.
I ain't losing any sleep over it either...though I also live in an area where supply isn't a problem, if I lived in the South east I might sweat a bit though!
Ian