My judgements on length is like this.
We have 16 chapters. They're not all the same length, they fluxiate.
The Game Informer dude said it took him 4-5 hours to best the first five chapters in the game.
There is an achievement in the game to beat it in under 5 hours. You can best almost every RE game in under two hours, and some games, like REmake, even gave you a reward for beating it in under two hours.
Time with these games vary greatly, if you explore, go straight along the path, go slowly to let the atmosphere get in or you're scared, if you're booking it. Difficulty playing on, get stuck on a puzzle or what to do as little hand holding, etc.
The demo area I played, which is only a part of chapter 9 and not the whole thing, took me about 30-40 minutes to get through the first time, but on a subsequent run I could do it in 10 minutes.