From Outliers-The story of success, by Malcolm Gladwell.

The idea that excellence at performing a complex task requires a critical minimum level of practice surfaces again and again in studies of expertise. In fact, researchers have settled on what they believe is the magic number for true expertise: ten thousand hours. The emerging picture from such studies is that ten thousand hours of practice is required to achieve the level of mastery associated with being a world class expert in anything. In study after study of composers, basketball players, fiction writers, ice skaters, concert pianists, chess players, master criminals and what have you, this number comes up again and again.

For a martial artist that would equate to:

3, (2hr) classes per week, for 48 weeks per yr, for 35 years,

or for a traditional Budo organization - 8th Dan

Of course this does not address why some people get more out of their practice sessions than others. But no one has yet found a case in which true world class expertise was accomplished in less time. It seems that it takes the brain this long to assimilate all that it needs to know to achieve true mastery. This is true even of people we think of as prodigies- Mozart, The Beatles and Bill Gates for example. So keep track of your hours and of course to accumulate the hours you need to, As Sensei O'Connell always says, SHOW UP!