It all depends on how beefed up your characters are, what equipment you have, and how well you know the interior of the place. On my last playthrough it took me about two to three hours to get all the way through and get my third Iron Duke. I'm taking a hiatus from the game right now and I'll come back to it soon to get the remaining three Iron Dukes so all three girls will have only Iron Dukes as their accessories. I'm already unstoppable as it is, but I guess if I've already come halfway, there's no use stopping...