This is kind of a tough one. Normally on Fridays I do what is considered by many to be rather short sessions, like three to four hours. However, at the EOW mini-convention that I attend every year, we usually game all day, like from after breakfast until dinner time. But I'm not sure which of those sessions is the longest.
So, I'm talking about an eight or nine hour session with a break for lunch in the middle. That's pretty much the norm for EOW.
As a side note, EOW is coming up in October, and I'm supposed to be running a session. Time to get cracking!