Boy...how much to charge? How long's a piece of string?
If you were a window cleaner with a few years experience then I would suggest you would could expect to earn considerably in access of £200 per day, there are some that are doing £200 plus on residential work...but it takes years to build and hone a round to that kind of level.
It is no use thinking, "Oh, I want £250 a day, this job will take me all day, so I'll charge £250."
As a window cleaning newbie it may well take you all day, but to someone with years of experience, such as myself it might only take two or three hours, your price would then appear well over the top.
If you are WFP then an average size window should only take you about 30 seconds, and that should be to a good standard too.
The trouble with WFP is that any newbie could flash the brush over the glass and claim he can clean a window in 30 seconds!!
With commercial work there is also the type of work to consider.
Small medium and large, the larger the job the less pro-rata per hour you would earn, though that would still equate to very good day rates.
If this guy knows and trusts you, perhaps he would tell you how much he has paid in the past and you can work it out from there, at least on one job!
You could then get a feel for what is being charged, but of course how much you earn then depends on how long it takes you to do the work.
Ian