I think a game demo should only be long enough to get the player interested in playing more of the game and end at a really interesting spot 
perhaps only 10-15 minutes long.
If the demo was long and the player obtained the full version later, would the player be bored playing the same part of the game again? One possible solution would be to have a slightly different puzzle in the demo.
perhaps only 10-15 minutes long.
If the demo was long and the player obtained the full version later, would the player be bored playing the same part of the game again? One possible solution would be to have a slightly different puzzle in the demo.