I know the story is that Gracies randomly chose "10 years" to black belt to weed out the not serious people.
but there is pretty good evidence that it takes around 10 years to master any sufficently complex field.
just thought it was funny than a number picked almost out of thin air seems like it is backed by science. :)
http://norvig.com/21-days.html
"Researchers (Hayes, Bloom) have shown it takes about ten years to develop expertise in any of a wide variety of areas, including chess playing, music composition, painting, piano playing, swimming, tennis, and research in neuropsychology and topology. There appear to be no real shortcuts: even Mozart, who was a musical prodigy at age 4, took 13 more years before he began to produce world-class music. In another genre, the Beatles seemed to burst onto the scene with a string of #1 hits and an appearance on the Ed Sullivan show in 1964. But they had been playing small clubs in Liverpool and Hamburg since 1957, and while they had mass appeal early on, their first great critical success, Sgt. Peppers, was released in 1967. Samuel Johnson thought it took longer than ten years: "Excellence in any department can be attained only by the labor of a lifetime; it is not to be purchased at a lesser price." And Chaucer complained "the lyf so short, the craft so long to lerne." "