I would think the average user of WFP is using approx 1L per min.
Open windows are not always that much of a problem, if the customer is not around to close them, you can either reverse your pole and nudge them shut, or clean them anyway but be careful.
If your brush is a one with the spray jets contained within the bristles of the brush it isn't a problem. Not so sure about the jets that spray over the top of the brush head though
Yes, water will splash onto windows below, but its only water, no worse than rain, and there isn't detergent in the water to stain the glass below.
Ian