The principle is attributed to Malcolm Gladwell (who doesn't need any more free publicity--there's less to him than meets the eye) but the research is based on the work of K Anders Ericsson (here's a link to an accessible article.)
So what does correlate with success? [...] All the superb performers he investigated had practiced intensively, had studied with devoted teachers, and had been supported enthusiastically by their families throughout their developing years. Later research [...] revealed that the
amount and quality of practice were key factors in the level of expertise people achieved. Consistently and overwhelmingly, the evidence showed that experts are always made, not born. These conclusions are based on rigorous research that looked at exceptional performance using scientific methods that are verifiable and reproducible. (p.1)
Our research shows that even the most gifted performers need a minimum of ten years (or 10,000 hours) of intense training before they win international competitions. In some fields the apprenticeship is longer: It now takes most elite musicians 15 to 25 years of steady practice, on average, before they succeed at the international level. (p.4)It's interesting to put the idea to the test in this way.
The definitive reference is: Charness N, Feltovich P, Hoffman R and Ericsson K (2006) The Cambridge Handbook of Expertise and Expert Performance Cambridge; C.U.P. (Beware, it's 900 pages!)
I've also discussed the idea here, here, and here.