I think it's difficult to give a general answer to the question, it depends so much on the game, and the players. As I'm getting older I find that I am drawn towards games that allow me to feel I progressed with something in a shorter session (say 15-45 minutes), even if the scope of the game is longer. But at other times in my life I've enjoyed games that allow for long stretches of playing. For the latter type, I think varying the intensity of the experience is important.
As for the game I'm working on now, I'd say a typical session is about 15 minutes, but hopefully there is enough interesting variation and progress to make it replayable and worthwhile to come back many times. :)