The demo missions, as good they were, were a little on the short side. I guess the first few missions in the game are about the same length as the demo missions but as the game goes on things get a lot more complex. The demo missions are also designed to be the shortest in the game, since they are meant to be the easiest.
Most missions are larger in size and complexity.
A little on the short side . It takes me hours and hours to get through. I need a nap after playing it. Multi Player should keep people playing long after Single Player is done with, tho. In fact there are many different ways of playing every mission.