This is a good read on the 10,000 hour myth. I can't remember where I originally heard that the idea as pushed by Gladwell has been debunked, probably the You are not so smart podcast. I've no personal insight to say whether this is a rule or a myth but the original researcher seems to disagree with the popsci understanding of it.
A research paper authored last year cited a study which found that only 30% of the variation in performance could be attributed to deliberate practice.
http://changingthegameproject.com/the-10000-hour-myth/