Extract from Malcolm Gladwell's Outliers: Is there such a thing as pure genius? | Books | The Guardian

"'In study after study, of composers, basketball players, fiction writers, ice-skaters, concert pianists, chess players, master criminals,' writes the neurologist Daniel Levitin, 'this number comes up again and again. Ten thousand hours is equivalent to roughly three hours a day, or 20 hours a week, of practice over 10 years... No one has yet found a case in which true world-class expertise was accomplished in less time. It seems that it takes the brain this long to assimilate all that it needs to know to achieve true mastery.'"