I had the demo first, believe me it doesn't even begin to give the game justice. As for length, I suppose the difficulty of the puzzles could be it, but I started playing BS1 a month before Christmas (in preparation) and it took till the middle of January to complete. That's when I started playing BS3, it took about 3 weeks, and only playing a couple times a week as opposed to every day, so that's how I judge. I haven't played the second one in a couple years, and I don't remember how long it took to beat so, can't use that either.
Just because people think what I say don't make sense, doesn't mean I don't like popsicles.