Don't charge by the hour!!
Break down windows into a unit cost.
Then it is just a case of walking around a job, counting up the windows, breaking them down into what you consider to be a unit and multiply by whatever you have allowed as your unit cost.
It makes pricing far more accurate and takes out a large element of 'guess-timation' from the equation.
If you are fairly new to the game then you need to know what the average time it takes for an experienced window cleaner to clean your average window.
In basic terms, an average casement window, about 45" tall and about the same in width, with 3 panes of glass, one narrow opening light above a single fixed pane, and a longer opening pane to the one side will take approx 90 seconds to clean, including any detailing thats needed.
And that is to a good standard including the sills wiped down properly.
Oh, the above is for trad window cleaning, but generally, even if WFP, it is best to price up as for trad.
AS Simon said, to begin with you will be miles slower than someone experienced, but these are the people you are pricing against, so you need to be competitive...
AS your skills and speed increase then so do your earnings.
What you charge per window (or unit) is up to you, and is also to a degree governed by your location in the country.
My own unit charge is £1.00, and my rate - per - minute - worked is also £1.00
This does not mean earnings of £60 an hour...not by any stretch of the imagination...there is a world of difference between the rate - per - minute - worked and what you eventually earn per hour.
The rate per minute is the time taken when you are actually at the windows cleaning them, no allowance for talking to customers, setting up or putting away or driving between jobs, or time off because of the weather/holidays/sickness/breakdowns and so on.
I'm WFP so my time per window is more like 30 seconds rather than 90 seconds and an average semi will take me around 10 minutes to actually 'clean'...but there is no way I will average 6 semi's an hour!!!!
Mr average on a good day will ...er...average about 3 0r 4 an hour over a full days work, oh, and for most an average days work will rarely be more than 6 hours before he (or she) is heading off home. That isn't to say many don't work considerably longer hours, we often work much longer ourselves (6am starts and 5pm finishes).
Something else to ALWAYS remember is that your hourly turnover rate (perceived not actual!!) is not your wage! it is your business income/turnover from which you take your wage....
Have a minimum charge and over and above that, price per unit and NOT per hour.
Ian