Leonardo worked on the Mona Lisa on and off for sixteen years, starting around 1503 and not finishing until shortly before his death in 1519. Walter Isaacson’s biography describes him doing the same on The Last Supper: staring at the wall for an hour, making one small brushstroke, then walking away — his own explanation being that the best creative minds accomplish the most when they appear to be doing the least, because their ideas are still forming. A 2019 clinical study proposes he had traits consistent with ADHD.