That time frame should be plenty of time. On normal taping days, virtually no one shows up before midnight, and you won't see more than a few dozen people before 4am. I'd say you should be able to show up considerably later and get in with no problems, but if you want to be safe about it (and get good seats, since the audience is seated in order of arrival), 5am should be fine.
I certainly wouldn't want to show up too late and risk not getting in at all, so I personally would show up as early as I could.
(That said, half the fun to me is getting in line at 2-3 am and chatting for hours with others who are just as into the show as I am. (Well, okay, obviously they aren't quite as interested as a guy with 5200+ posts on this website, but you get the idea.))
Also, another random thought...most of the people camping overnight (like, showing-up-the-day-before camping) were doing so for Bob's final year. That's obviously not going to be the case today in 2008.