Herbie,
That is one of those how long is a piece of string questions! (the answer to which is; twice as long as half it's length and always shorter than what is required to wrap a parcel
))
An average semi with 10/12 UPVC windows will use approx 15 to 18 litres of water on a regular repeat clean.
You will use more on a first time clean, probably a little more on painted windows.
So it will be about 1 & a half litres for an average window.
How much the average WFP'er will use in a day will depend on how many windows they clean.....depending on their flow rate of course
Which is the other way to work it out
regards,
Ian