One important thing is self will. I never thought about making a dream longer or shorter, just staying aware longer. When I was attempting in this manner, I have once stayed lucid in varying degrees, through six dreams. Between dreams, all I saw was my face in the dark. This happened between each dream.
Generally, I dream for content, or exploring and going over my areas of awareness--sight, sound, taste, feeling, etc.
Content comes, for me, by concentrating on a very specific question befor entering the lucid dreamstate. The length of the dream is of no importance compared to the content.