Pops as soon as the credits are done rolling. This is just like the 'An Old Friend Returns' achievement for AC2 (which I would think most people played first).
If you haven't played AC2 yet -- essentially you'll have to watch the list of every Ubisoft employee who had any impact on the game whatsoever, down to that one guy that brought everyone coffee that one time. There is no way to fast forward, and you probably want to stay close because your controller will shut off at one point (mine did as I was typing this while I waited), which will pause the credits until you turn it back on and hit contrinue.