On the assumption that not all windows would be done from the cherry picker, I'd go with 2 guys 1 day.
Allow double for the first clean.
I never send my guys out solo, it's part of my risk assessments.
We always need at least 2 guys 1 of them being a first aider. If it means that 1 of them is stood about kicking his heels all day then that's the way it is.
You can put that in as part of your quote that's what I do, particularly if they have asked for risk assessments, if not I would submit some any way as it looks good and you'll get away with charging more.
i think its going to take a lot longer than one day, average time per window including moving about, set up, lunch, etc, is about 3 mins per window x 900 windows = 45hours! so about 5 1/2, 8 hr days and thats a very optimistic estimate, theres always something that will slow you down,
i remember giving a quote for some high rises cant remember the exact number of windows, but it would have taken about 10 of us at least a week to do, quoted him £20,000 every 3 months, he said he's had a quote for £3k! that wouldnt have covered my insurance! i dont know how some people make a profit!