How long does the copyright for a game last, after it is out of print?
As previously pointed out, just because a copyright holder has stopped producing a game doesn't mean they stop owning it.  GW still own the rights (unless they have sold them; you could write and ask them if this is more than curiosity).  Games are usually covered by copyright in the rules, characters and concepts though there may be patents on mechanics; whichever feature you are specifically interested in, there will be a law stopping casual copying unless GW's lawyers were totally incompetent, which seems unlikely.