How fast you can go depends on several factors, the type of window, your experience, your confidence, system, flowrate and so on.
But there are averages you can compare yourself to.
If you are trad, then by and large a standard 3 semi will take you about 20 minutes from pulling up to driving away, that is for about 12 normal casement style windows.
WFP should be about 15 minutes tops for the same thing.
Something that has never really changed over the years is the fact that a normal, 3 pane (one opening light, one longer opening pane and one large fixed pane) window will take about 90 seconds to clean, including full detailing and wiping of the sill, done trad of course.
WFP the same window should take you no more than 30 seconds or so.
Ignore the claims of those who can trad a window in 20 seconds or clean 10 houses WFP in an hour, all may well be possible but you need to know what you can reasonably expect to do.
I would personally expect to take a maximum of 10 minutes (WFP) for a standard semi, but that time would increase depending on things like access or badly oxidized frames and so on.
Knowing the kind of times it should take you will help give you target times to aim for if you are way off the average times.
As you familiarize yourself with all of your accounts, you begin to know the ones where you can either fly through and do a top notch job and those that you have to take more time to achieve the same result.
Ian