Ooo
For me that would depend on how many panes of glass per window and also the size of the panes.
My own house has 13 windows, including a half georgian door, with the WFP it only takes me 5 or 6 minutes, tops.
The old way with ladders and squeegee took me about 15 or 20 minutes.
I know it also depends on the part of the Country you are in, but I would have a hard time justifying charging more than £8.50 or so for a bungalow of that size. Unless all the windows are geogian of course
Or the windows that are there are big plate glass affairs
But if you can get that sort of money then more power to your elbow!
Ian