How many windows can one man clean in a day, using this method?
Its really down to what kind of windows and the difficulty in reaching them much like any other window cleaning
How to size the building up would be by drops, most abseilers will drop straight down from the roof and get as many windows as they can reach dependin on how the windows set up and what the H&S allow, a good rule of thuimb is a guy can cover 3 times the width of his outstretched arms although local rules may not allow that
So stand back from the building and count how many vertical lines of windows there are and that should give you the amount of drops
On 11 floors and 248 windows, that should mean about 23 drops unless the windows are real close together, if they are then maybe 12 drops
everything being perfect and a good work day it may be possible to finish in a day but more than likely 2 days for 2 guys, but something like an outcrop at the top or difficult windows could increase the time by a lot
If you have a picture post it and it would give a better idea