How much approximate gameplay is there in the full version of Limbo?
There is an achievement for completing the game in one sitting, with 5 deaths or less, so I would think that for the standard player, it shouldn't be very long. I completed it in one sitting myself, and it took me between 2 and 3 hours. for 1,200 points, I'd expected a bit more, but the atmosphere and ambiance in the game mostly made up for my feelings of being shortchanged.
According to this Gamastutra article, about 4 hours, up to 5 or 6 if you get stuck on some of the puzzles.