This interview on GameSpot reveals demo details:

GS: How long is the demo to play through, approximately? How difficult is it compared to the other levels in the game? Are there any hints or tips that you feel you should give the audience at this point?

MG: The demo will probably take the average player about 20 minutes to play, although there are several hidden items throughout the level which will take the player much longer to find. It's pretty early in the first level, so we're not too rough with the player at this point. The puzzle by the waterfall is a three-stage puzzle, but you really only need to complete one stage to reach the end of the demo. In order to finish the demo, you don't need to climb to the top of the waterfall, but I encourage players to go up there first, as there are several goodies hidden there. Also, who doesn't want to swan dive off the top?