Mine first convened in October 2004 and last played in June 2006, putting us at the low end of "More than 18 months".
Due to some long stretches where we couldn't play every week, or even close to it, along with a large group that is somewhat easily distracted by shiny things (though nowhere near as much so as some others I've run), we only made it to Lords of Oblivion, I think - whichever one is right after Test of the Smoking Eye. I was using the magazine versions at first because the hardcover didn't exist yet, so we also skipped Drakthar's Way and, as a result of the party being a bit underleveled for it, part of Flood Season (as soon as they had the minimum required number of the gemstones I replaced the wands with, they cut and run, probably wisely).
I am cautiously optimistic about continuing the game online using Klooge Werks or something like it (I now live in a different city than the other participants), but so far it hasn't happened and I know it won't until after Christmas at the soonest.