I prefer to measure games in days entertainment and like most action/adventures of it's time I'd typically spend seven to ten days completing it pretty thoroughly. But that would include plenty of playing around as I went. Last time I played it I spent a session jumping off places just to see how far and where I could get Raziel to glide to.
I play typically in 2-3 hr sessions so I reckon that means a straight, first time, (guide free of course) playthrough would have taken me 15 - 20hrs.
I've read other say 12 -15 hrs so I'd guess-timate 15+hrs as in my experience many gamers feel the need to exaggerate their first time speed through a game.