It took Leonardo da Vinci four years (1503-1507) to paint the Mona Lisa.
This was long enough for his patron, Francesco del Giocondo, to get impatient.
Giocondo had commissioned the portrait of his third wife, Lisa, but enough was enough.
Giocondo refused to pay for the unfinished portrait, and Leonardo sold it to the king of France instead.
This is the traditional tale, but it is based on sparse evidence.