[The real question there is "How long have other people been calling you that?" He's been calling himself by that name for as long as he's had the idea in his head to be a pirate. It took a little longer for it to catch on with everyone else...]

For as long as I've been sailing. Those who looked upon my sails learned quite quickly what sort of man they were up against. It wasn't so long after that that my name and reputation spread throughout the land. Why do you ask?