There’s a fun study about that which compares great violinists to elite violinists. http://graphics8.nytimes.com/images/blogs/freakonomics/pdf/DeliberatePractice(PsychologicalReview).pdf
The gist is this:
The time diaries revealed that both groups spent, on average, the same number of hours on music per week (around 50).
The difference was in how they spent this time. The elite players were spending almost three times more hours than the average players on deliberate practice — the uncomfortable, methodical work of stretching your ability.