Leonardo's painting of Ginevra de' Benci was painted shortly after her wedding to Luigi di Bernardo Niccolini in 1474.
Part of the painting has been lost at the lower edge, and it is believed that a silverpoint drawing of hands by Leonardo is a study for the missing part of the panel. Vasari describes Leonardo's portrait as dating from about 1500-1506, but the sitter would have been in her forties and this seems unlikely. However the work would appear to be more accomplished than some of Leonardo's work from the same period, so the exact date of the painting is open to question.
It should be noted that Da Vinci did frequent the Benci household and was a friend of the sitter's brother Giovanni. This was before Leonardo left Florence for Milan in 1482 and points to the portrait being painted before this date.