There is no a specific moment of time that needs to be passed for something historical to become a legend. Legends are sometimes based on historical events, but because of the lack of information and evidence that those things really did happen we simply believe that they are made up stories by the people from the area.
History ends and current affairs begin at the point at which consequence is unknowable, documents are unavailable and at which moment reaction rather than memory informs oral testimony. In Britain an obvious definition of that point would be the 30-year rule on the release of information.
