It depends on your server hardware.  Just kidding , but hardware an RAM definitely has an impact.  Dont' know if this helps you or not, but I have a 16.5 Gig user DB on a 2 X 2.4 gig CPU server with 2 gigs of RAM and DBCC CheckDB's complete on this in about 7 1/2 minutes.   On the same server, CheckDB on a 10 gig database completes in about 4 minutes.  Not sure if there's an explicit way of determining this without actually running the command but I'd be interested in finding out if there is.