I was in Treasure of Tierra Padre with one of Xael's characters. It started in June of '04, ran for 4.5 months, then stalled when 3 characters dropped out. In game, the action moved back to the RDI thread, where several new characters (including Xael's) were recruited. We then went back to the Treasure of Tierra Padre thread and finished up in another 7 months. So, the whole thing wound up just a couple of weeks short of lasting a whole year.
I have characters in 2 adventures now, Curse of House Thimbrall (which has been going on since April, or 4 months now), and Festival of Halina (since June, or 2 months). The same character that was in Tierra Padre is the one in Festival of Halina. The handoff between the two adventures was interesting because Festival of Halina "formed up" in front of the Inn the day after PBartender released our characters from Tierra Padre! I'd expected Halina to take a few days to get started, and was caught flat-footed without xp or loot being awarded from Tierra Padre. Bront was pretty gracious about letting me edit my character sheet to "level up" during the first few days of Halina, and the two DMs handled the loot distribution in-game in a clever way that made ENWorld seem that much more like a real place, with couriers riding forth between the RDI and the caravan my character was in.
I think the record for longest adventure in Living EnWorld is probably held by Heroes of the Vesper Peaks, DM'd by Creamsteak. It started in April of '04 and is still running as of a couple of weeks ago, or 16 months and counting.