The longest tournament was live, when we organized a tournament with friends and acquaintances, but we couldn't finish it for 4 days because everyone has a job and a family
I had a tournament that lasted eighteen hours, it was a tense and tiring session, a state of rigidity, but then I managed to take part out of twenty-five thousand people, to take first place.
Longest one day online tournament for me twelve hours and forty five minutes. I have played some multi-day online tournaments but those are managed so you don't end up playing that long.
The longest time I participated in an tournament so far has been for 5-6 hours and managed to get +/- $100 dollar. The tournament lasted eventually 7 hours on partypoker.