ScienceDaily (Oct. 24, 2011) — Psychological scientist Guillermo Campitelli is a good chess player, but not a great one. "I'm not as good as I wanted," he says. He had an international rating but not any of the titles that chess players get, like Grandmaster and International Master. "A lot of people that practiced much less than me achieved much higher levels." Some of the players he coached became some of the best players in Argentina. "I always wondered: What's going on? Why did this happen?"
Now a researcher at Edith Cowan University in Joondalup, Australia, Campitelli studies practicing. He's written an article with Fernand Gobet of Brunel University in the United Kingdom on their and other people's research on chess for Current Directions in Psychological Science, a journal of the Association for Psychological Science.
In one survey of chess players in Argentina, Campitelli and Gobet found that, indeed, practice is important. All of the players that became masters had practice at least 3,000 hours. "That was not surprising," he says. There is a theory in psychology that the more you practice, the better you'll do in areas like sports, music, and chess. "But the thing is, of the people that achieved the master level, there are people that achieved it in 3,000 hours. Other people did, like, 30,000 hours and achieved the same level. And there are even people that practiced more than 30,000 hours and didn't achieve this."
http://www.sciencedaily.com/releases/2011/10/111024153448.htm
3000 hours is probably also a good rough estimate of the time it takes to become fully competent at an occupational skill that is within one's ability.