Rooms or floors that don't get used don't need to be cleaned very often. Dust accumulation is basically nonexistent in the absence of human activity. If the building uses anything other than forced hot air for heating the dust accumulation will be lesser still.

A couple janitor would have a commercial building that size cleaned in a night of work. There's no reason the owners can't use the same techniques one Saturday a month.

Yea, at 20000 square feet...by the time you finish cleaning the entire thing, you'd have to start over, since where you started is already dusty.