It's pretty much as long as you make it.

You can just rush through the bosses and be done if that's all you want to do. That'll probably take you a day or two max, depending how much time you have to play and your general skill level.

But if you're interested in looking around the world, building, gathering, crafting and not just going from boss to boss, you can get anywhere from dozens to hundreds of hours of entertainment out of this game.