I agree with the anonymous Irish player mentioned above in connection with Carthy, but I'd rather say "you've got to love the music enough to explore (play) it a lot" rather than use the word "practice", which has (for me) too strong an implication of mere mindless repetition. ("Strive to play the maximum number of notes per second. That way, you gain the admiration of the incompetent." - anon)

From an article by Ericsson, Prietula and Cokely:

"The famous violinist Nathan Milstein wrote: "Practice as much as you feel you can accomplish with concentration. Once when I became concerned because others around me practiced all day long, I asked [my mentor] Professor Auer how many hours I should practice, and he said, 'It really doesn't matter how long. If you practice with your fingers, no amount is enough. If you practice with your head, two hours is plenty.'"