[geoffreyspecht]

sixty windows is three hours work of ladders  of ladders  so u should charge at least  £60

[Paul Coleman]

I would say 1 window hole (in the brickwork) if its large then its double

I do it similar to that as well.  I think of it as £1 per climb for upper floor and £1 per window for ground floor.  If I need a ladder for ground floor too I might charge a bit more.  Also, if there are a few very small windows on the ground floor I might think of two or three of them as one window.  After assessing it like that, I also take a step back and ask myself how long it's going to take me because pricing by windows will sometimes leave a quote too low.
TIP:  Never tell the customer that you price per window.  Some of them will assign you all the difficult ones and do the easy ones themself.