Check out the Games
section relevant to your area and see if there is anything coming up in the near future. The host generally establishes a context for the game before-hand and describes it in the game post.
Some games may last only 3 or 4 hours and consist of multiple short scenarios while others can sometimes run up to 24 hours with the right group of people. There is a game this August in Ontario that is set to go for 30 hours.