Haha, maybe I am seeing this wrongly, but it seems like Dark is trying to get us to buy the Overture game.
If you took time to explore the environment, and really try to 'get your money's worth' out of it, I think it is fair to say that it would take at least an hour...or, if you were scared out of your wits the entire time (such as I) you would proceed more slowly, making it even longer.
The way I see it, the tech demo was actually spookier for me (mainly because I saw so many Overture trailers; I knew what to expect in many occassions...the tech demo plops you in the middle of things, and has no weapons...save the dynamite). If you enjoyed the tech demo, then be sure to purchase Overture, for the engine is updated (better physics, graphics, etc.), although in my opinion, it could still use some patching up...being that physics is the hallmark of this game, I find it ironic that the engine has so much trouble running through little things, like pencils/planks of wood...of course, a patch would likely fix any problems, as would an updated engine in a future release (think Episode 2).
EDIT: When Dark says "Ignore the tech demo from now on" I think he is referring to the fact that you should disregard the engine, for it is better in Overture than in the demo...is this the case? I just interpreted it as being 'salesman' talk a little while ago...must be me.