Currently Online 
Barozi on 25 October 2009
I've completed all main and side quests and made a few other things that gave me gamerscore. Took me about 56 hours.
I would estimate that the addons will last about 18 hours or so (only completed Operation Anchorage & Mothership Zeta so far)
So ~75 hours I guess. If you're going for platinum trophy I would say 80 hours.
