Less than a Gig for basic stuff, but once you start playing with cvsup and adding to the ports collections and src tree it will start growing! I am using 2.8Gigs
and I am mostly using it for browsing, E-mail and music/movie playing. I am sure it is not a clean as most people who know what they are doing have it though. We have some nameservers with OpenBSD taking only 100M.