Good question.
I'd say that for every serious scene that lasts over a few minutes, there should maybe be a portion of the game where a character tells a funny joke, or the game has a minigame that's based on getting the player to relax (fun card games, etc).
I actually feel a fairly good example of such a design choice in action - would be games like Final Fantasy VII or Mega Man Legends.
I think it is generally easy to give the game a rhythm in the first levels, the real problem comes later, in the more advanced levels...
It is something difficult to test for the creator of the game, since you have done everything, you will not feel it in the same way as the player...
When I play my own game, I don't last more than 30 minutes, but because nothing surprises me...
