For world class chess, it might be 50,000 hours. This gives insight into specialists versus generalists. It also suggests the need to teach for transfer and to select topics that, counting their transferability, have broad importance. I need to give more thought to the issue of knowledge and skill being embedded in a tool, and perhaps the tool including built in CAL just in time instruction on how to use the tool in a situation that is out of the ordinary or basics that a person has learned. A computer tool can be designed to facilitate transfer.
Human children have a long childhood.
During this time their brains develop and they learn a great deal. Many of the threats and opportunities of today's world are quite different than those faced by our species when the brain was evolving to its current capabilities. Many of the problems and tasks that people currently face require a high degree of precision and correctness. The human brain evolved to make relatively quick decisions and to take quick action. It is an analog problem solving and task accomplishing aid to survival of the species.