This depends on many things:
Location
Distribution of work (domestic/commercial)
Average speed
Areas worked
"Full time" is a difficult concept to apply to window cleaning, as it is weather dependent. No-one can work 5 days a week unless they also have indoor work and this is relatively rare. Also, many window cleaners (not me) stop working at 2 or 3 pm.
I'd say, if you lived in the south of England, were quite fast and priced your work properly, worked from 9-5 with 1hr for lunch, a daily average of £140 is easily possible.
You can usually depend on about 4 days of good weather per week.
Assuming all these things, and that you had enough work, you end up with an annual profit of (£140 x 4 x 52) = £29,120 before tax.
(This would assume no holidays or sickness though, and unlike other jobs you do not get paid for that time)
However, if you have mainly commercial work you might earn closer to £240 per day. Also if you use a pole system the speed/daily rate might be greater still. Inside work you do for 5 days.
Anyone any other thoughts?
Silly