Insides to going tobe harder, rregardless, you say its a business centre so will it have obstacles like desks etc in the way of the windows. The outsides'll take about an hour as a regular clean. First time is likely to be different.
Without seeing the place, Id allow half a day for the lot.
Actually, without seeing the place, I wouldnt even contemplate a price, but half a day is the place to START working from. Havent you got any pics?