Answer:
There are no writings by Leonardo or Vasari stating it took 10 years to paint Mona Lisa's lips. Giorgio Vasari, Leonardo's biographer, states Leonardo spent 4 years on the whole painting. For up-dated information regarding the Mona Lisa check out, Mona Lisa, Inside the Painting by Abrams. It is the result of one year of unprecedented in-depth study of the painting by the Louvre.