Yeah, Satantango is something of an endurance. When people talk of '6 hour versions' though I always imagine they just mean a rough cut, or a workprint, rather than an exhibitible version. I could imagine the long cut of a film as montage-heavy as Tree of Life easily running 6 hours.
The longest film I have seen is Jaques Rivette's Out 1, which runs about 12 hours in 8 parts. Although this seems like a lot, it really isn't if you compare it to a season of a TV series.
Satantango is something of an endurance test. All I remember is a scene of the drunk people dancing in the bar, and later someone watching that through a window.
Is Satantango the longest theatrical narrative fiction film that was intended to be seen in a single sitting? You will notice that description excludes a) artist installation things b) film series c) documentaries d) anything made for TV or intended to be shown on TV, even if it wasn't (e.g. Berlin Alexanderplatz, Heimat, Out 1)