Well, within a few days of hearing it. It helps that it's a narrative ballad and each verse follows (fairly) logically from the one before. I also know a couple of other versions and used to sing the one learned from Tim hart and Maddy Prior (A wager, a wager, a wager I will lay, I'll lay you five hundred to one.....) I'm also cursed with what a fellow singer at Wimborne calls a 'prodigious' memory.