In a four and a half our session, a new party that hadden't played together managed to get through a little over a third of the module. If they'd known each other and were familiar with my Dmin'g style, I'd say they'd have gotten through half easily.
Really efficient adventurers may knock it over in one eight hour sitting, but I'd say two to be on the safe side. Few 1st level parties work particularly well as a team.