Not long at all. The stories are there (from the writers) just for dramatic effect. Not really needed part of the game:
http://www.imperialconflict.com/forum/viewtopic.php?id=145042
^ That's a game we just completed. It's quite fun.
Essentially the game is like this (with obviously a few things different here and there):
- Two factions (Town and Mafia)
- One faction wins, when they're the last ones left!