Hi ICC;
This though probably comes after-the-fact, but I hope it helps for anyone else wondering the same thing.
What I would do is clean a room in my own house approximately the same size as the one your client wants done. Clean it how they want their office cleaned. Time it. Multiply that time by how many offices. If you charge an hourly rate, you can get a general idea how long it will take. For those who've been in the biz awhile, they can automatically estimate based on just a visual and knowledge. For the showroom part of the office, that's up to you. A note: I always estimate a bit higher. The client will be happier if you quote high and finish below that time than if you passed what you estimated.
I'm confused... :) But in a knowledgeable way. :)