I voted for two hours. I've polished off a 24 pack in a hard night of drinking, but that was a 6 hour period that also included hard alcohol. I've put away 12 packs in 3 hours; if I were legitimately trying for speed, 2 hours wouldn't be difficult. I'd feel like a bloated sack of shit, but I could do it.
As for people saying "no one can do 12 beers in an hour!," you're dead wrong. I personally watched my good friend get through 14 in an hour just to test his speed. I know some guys who are built like linebackers (6'4", 280 pounds) who can drink a bottle of jager in an hour and be basically sober. Granted, that's a whole hell of a lot less liquid, but they could probably do the beer challenge in an hour easy too. Big American men who are used to eating 6,000 calorie meals are going to be fine consuming that much beer.
-EDIT- I may have to try this over the weekend and post my results... Hell, maybe everyone on ATOT should. Then again, we probably shouldn't encourage people to drink themselves to death...