There is a 22h "speedrun" for final fantasy 6 no sketch run. Also somehow a 16 hour run was accepted for mario 64. That is probably up there.
https://www.speedrun.com/run/pydo1kvz to link it (you beat me to it by 1 min)
Or this would have given you the result if you used 2 min of your time http://lmgtfy.com/?q=longest+speedrun
Really all you have to do is start any run for any game, leave it running, and finish a few weeks later
The FAQ pastebin for Baiten Kaitos 100% is here (on the same page as the run btw). http://pastebin.com/BRvPJ430
Today I learned that people still use this thread to get the reply to this question, because it appears in the top results of Google for example. So I'm gonna make a reply because holy heck things have changed in 3 years.
First, the Baten Kaitos 100% World Record is now 338 hours 43 minutes and 26 seconds to this day. BUT in case it gets improved and I didn't update this message, I'm going to link the 100% LeaderBoards and not the link of my current WR: https://www.speedrun.com/baten_kaitos_eternal_wings_and_the_lost_ocean#100
Second, there are longer runs than this, even if they are debatable. The one that comes to my mind immediately is for the game Spider: Rite of the Shrouded Moon, with the category 100%, that is 528 hours long https://www.speedrun.com/srotsm#100 (But personally I would call this category "100% No Time Manipulation" because as the name implies you cannot change the clock of your PC for this category, and that makes it debatable because it's an added constraint. And if Time Manipulation was a thing in Baten Kaitos 100%, it would then be much faster.) For more explanation about all of this: https://www.speedrun.com/srotsm/thread/yuvqx
And of course, don't even start with the Theoretical Longest Speedruns. There are hundreds that would be longer in theory. EZScape made a video about it:
A challenge defiend by the "longest time" isnt a speedrun though @Powerofdan
Botw 100% is also a very long run. The wr is 18 hours or something. Unless you just want to torture yourself, I wouldn't suggest running it
Digimon World 2 Any%'s WR currently stands at 16h 59m, but it used to sit at a comfortable 24h mark.
As far as I know, since I have seen no mention of this, the longest timed speedrun is Animal Crossing: New Horizons "Golden Rod, No Time Travel" in 6173:01:32
Soooo... yeah.
However, this type of run is not featured. The normal golden rod run uses time travel, so the WR must be somewhere around 6 hours.
hollow knight low% te is 30 some hours def not the longest but for what's barely a meme category (by title, content is pretty memey) that's fairly long