The Binding of Isaac is a deceptively long game if it's full completion you're after. I'm at 25 hours and I know that's nothing. Edmund McMillen recently started working on a new expansion, "Wrath of the Lamb," that will add more of everything (and extras) while retaining the game's structure.
McMillen has smartly said this three-dollar expansion will be done when it's done, though he suspects we won't get to play "for months." Truth be told, many of us won't need anything else to go on besides knowing this is an extension of the same fantastic game.