Funny thing is that making lengthy prose or poetry into song (i.e.; linking it to a melody) makes it a lot easier to commit to memory. Well, at least for me. Music as a mnemonic - what a concept, eh? And then there is Seamus Heany's comment about Beowulf - "It was meant to be heard, not read," or words to that effect. There are some folks about who will argue endlessly about the proper length of a song, some in terms of time and some in terms of verses. I don't think there is a "correct" answer. It seems to me the only wise course to read to your audience as you go. If they stay with it, take it to the end. If they seem to lose interest, cut it short.