From Outliers-The story of success, by Malcolm Gladwell.
The idea that excellence at performing a complex task requires a critical minimum level of practice surfaces again and again in studies of expertise. In fact, researchers have settled on what they believe is the magic number for true expertise: ten thousand hours. The emerging picture from such studies is that ten thousand hours of practice is required to achieve the level of mastery associated with being a world class expert in anything. In study after study of composers, basketball players, fiction writers, ice skaters, concert pianists, chess players, master criminals and what have you, this number comes up again and again.