What you say is true as far as it goes. It also depends on the height of the ceiling, because it's actually about a volume of air rather than a surface area of

What you say is true as far as it goes. It also depends on the height of the ceiling, because it's actually about a volume of air rather than a surface area of floor space. What you really want to know is how long it takes to recirculate the entire air volume of the space you're interested in. But, for a typical dwelling that has ~10-ft ceilings and isn't too leaky, that square footage number is a really good guideline.