Depends on whether you just want to see an ending, or whether to do everything...

If you just want to see an ending, and you know where to go, you can probably finish the game in about 5 hours, or even less maybe, at between level 40-60.

If you want to do everything, as in all items, bosses, covenants, spells etc it will take you countless hours and probably three new game pluses. The game keeps going forever if you want it to.