Title says it. This includes games with multiplayer on the side but the focus is on the single player.
My shortest has been Dino Crisis 2. I've finished it so many times in the past that I've been able to notice an average completion time of a mere 3h20m. It's quite a fast paced game in comparison to Capcom's other titles. I remember the first Fighting Force game to be quite short too.
I know this isn't a very productive topic to discuss, but so what? It's interesting to know which games have terribly short SP, probably so that you can know to avoid them completely at any point if you're looking for a substantial game. So please share your experiences. Not so much the game was actually GOOD or not, though. Just if it was painfully short.