Just completed the game, I'd say it took me around 7 hours.
Im a bit torn about the game to be honest. I did enjoy a lot of it but it kind of felt like there was a good game hidden inside somewhere but there were too many issues to ignore. My main problem was the story. It was almost like the dev thought up this whole backstory and world but then forgot to include loads of it. I spent a lot of time having no idea why I was doing what I was doing which is a shame as I really liked the characters and found the world intriguing.
...Also, fuck forced stealth sections with automatic fails!
Still though, its a gorgeous game. Not sure I've actually seen better. If a sequel does get made then I would be interested to play it. Hopefully they take on board a lot of the complaints.