It's not the longest, but my favorite LONG ballad is "Montrose." Done by Steeleye Span (a tour-de-force for Maddy Prior) on their record "Live At Last" (and it IS a live performance). I just timed it at a few seconds short of 15 minutes. It tells the story of James Graham, 1st Marquess of Montrose, covering pretty much his entire life from boyhood through his betrayal and execution and is full of historical references to real people and events that are remarkable for their accuracy, (which I appreciate). I counted 42 verses (including choruses) sung to 3 quite distinct melodies and includes several short instrumental interludes. If you have an interest in Celtic history, don't miss this.