I have just begun to read Elijah Wald's latest book "Jelly Roll Blues: Censored Songs & Hidden Histories." He begins by talking about a recording Jelly Roll Morton did for the Library of Congress in 1938: "The beginning of the song was not preserved, but what remains lasts at least half an hour, filling seven sides of four 78-rpm discs."
Even with Jelly Roll's marvelous piano playing, I'm not sure that I would last that long.
On the other hand, Martin Carthy's 11-minute "Famous Flower of Serving Men" seems not a minute too long to me and hearing say, Mike Waterson, sing "Tamlyn" for over ten minutes is not a chore either.
So, definitely depends on who is singing.