Yes, 8 or 10 hours. This game is big. I've still only opened up about two-thirds of the map in that time, and I know there are a TON of secret areas I haven't touched (I've found two or three so far but there are many many more). I'm sure it's possible to "win" in less time than I've already spent, if you beeline to objectives and skip over side quests, but man you'd be missing out.
Also of note: the storyline is very weird, and the characters are really full of tropes, but it's still incredibly entertaining. I'm finding myself fascinated by the world and the question of how it became the way it is, and of course science vs. religion is always a fertile germ to grow a world around.
Everyone should try the demo. If you like it even a little bit you should grab the game while it's on sale. The demo gives you a taste, but there's more to the actual game, as you gain a crucial ability:
Give it a try.