To be honest, I really don’t remember. I’ve been playing role-playing games since 1983 and that’s a lot of game sessions. I know that sessions used to be longer when I was young; as age and time commitments increased, average game session length decreased. Moreover, a lot of streamlined rule sets now allow intense gaming experiences in as little as a couple of hours.
Would weekend-long LARP sessions count? That would have to be my longest, I guess, and they date back to the mid-1990s with the Adventurers’ Inn.