Gawd let me put it plainer so you understand...if you have your audiences attention you have it you have them engaged, you have them enthralled, I would hope, with the tale you are telling, but if you lose that attention, you may as well give up and go home. Unfortunately there will always be those whose attention doesn't go beyond the two and half to three minute mark, they would have found The Animals version of House Of The Rising Sun (Four and a half minutes) too long and Martin Carthy's rendition of The Famous Flower of Serving Me (9 minutes) well that would be well nigh impossible.
For me, personally, if the narrative of the song, regardless if it's trad or not, grips me then that's what it does, if it doesn't then it doesn't, the length of the piece is irrelevant.