Currently listening to Blaise Bailey Finnegan III by Godspeed You! Black Emperor off their EP Slow Riot for New Zero Kanada.
This is the kind of art metal that makes Tool look like they just aren't trying hard enough. Anywho, this track is 17:45 long, making it the longest single song I have on the old iPod. I've got a few "tracks" that are longer, but most of those are long remixes of existing short-form songs, or they were the last track off a CD with a hidden track attached to the end (and however many minutes of blank space were en vogue that year). I think the longest track I have is the final track off OK GO's Oh No!, which, full disclosure, I haven't listened to yet, so it may invalidate this whole posting, but I think it's a safe bet to hedge. That track is over 34 minutes (longer than most Sum 41 albums), and just behind that is the final track off Jars of Clay's self-titled disc, clocking in at 27 minutes and change.
Of course, I don't have any Iron Butterfly, so perhaps none of this is all that impressive.