Lots of great posts on this, but generally the op does not have enough experience to take this on, not enough water or manpower what happens in the 2 weeks your cleaning this home to your regular customers ? - most places will not want you taking 2 weeks either.
Like ms dean said this is a two man job that may be done in a day at most day and a half, cleaning a large site does not compare to residential work in terms of the number of windows (size is a big factor for example office windows can be taller and wider than residential then there are glass atriums but offset that with they are all in a row (compact) a two man crew cover 200 meters without a single move - this could be easily a third of the building
Be prepared to take a long time on the first clean to get them up to a decent condition the following cleans will be quicker but I suspect the 3 monthly clean we be binned within a year for either six monthly or annually
Darran