Hi James, congrats on your first commercial quote.
First and foremost-try and get on with the caretaker/site manager or equivalent.
They may just tell you what they are paying/have paid in the past.
Or ask them if they have a budget? Again-it might near enough give you a price.
Quoting blind-if it's 2 floors you should have no trouble-quote it like a house and knock 10% off. Bigger windows are quicker to clean, especially if you fit a wider brush.
All the best with it