"If we're talking about "traditional folk ballads", those songs can continue for far longer than our modern day ears ( and brains ) are accustomed to. However, I feel that our shorter attention span is a result of our modern day input of T.V., sound bites, commercials, rapidity, etc."

"Traditional folk ballads" are the ancestors of TV drama, stories sung to while away the long winter evenings in the pre-electric age. They are, even the longest of them, much shorter than their modern equivalents.

In my limited experience, folk clubs, where they are not primarily antiquarian societies, seem to be more the descendants of the glee club tradition, rather than the homely fireside tradition.
My personal rule-of-thumb, therefore, when singing at folk clubs, is that a song should fit on a single A4 sheet when printed using 12-point Verdana typeface.