Well, when most people ask this question, they are really interested in an order of magnitude estimate -- because they want to figure out WAN bandwidth for doing remote subnet scans and whatnot. For example, should they expect 1 MB, 10 MB, 100 MB, 1000 MB, etc. per machine? I suspect the answer for probably all full scans on every type of machine is somewhere between 5 and 50 MB.
As for whether or not it should be in the FAQ, I think it should be -- because it IS a frequently asked question! Whether or not it can be answered well (which would be a FGA, Frequently Given Answer) doesn't affect its FAQ-ness...
