I usually have to clear an entire evening for a game (about five hours, from setup to calling it a night...teardown happens the next morning). That said, I've also played a couple of games for six hours to a status of "well, we're not
losing...but this could go on for another six hours." The game is incredibly unpredictable in length. Fortunately, I have some friends who enjoy it, and the cooperative nature of it means that my fiance is onboard with it as well, which is a lot of fun.
I picked up the Dunwich Horror expansion and like a lot of things that it adds. I've read that I should add the King in Yellow as well...maybe once my local game store opens back up.
...If it opens back up.