There isn't one best session length, for me.
I run 4 hour sessions, because I have to play on weeknights, and that's the time there is. In the past, I've run and played 6 to 8 hour sessions. For Paranoia, I've done 12+ hour sessions. For larps, I've played in weekend-long games, where the only real breaks are for sleeping.
It is all a matter of what the players like, and whether the individual GM is good at pacing a session of a particular length.
I run 4 hour sessions, because I have to play on weeknights, and that's the time there is. In the past, I've run and played 6 to 8 hour sessions. For Paranoia, I've done 12+ hour sessions. For larps, I've played in weekend-long games, where the only real breaks are for sleeping.
It is all a matter of what the players like, and whether the individual GM is good at pacing a session of a particular length.