I just returned from a jazz workshop where we talked a lot about practising. The drum teacher said that it's not necessarily the amount of hours you put into it, but also the frequency and how focussed you work on it.
I remember reading once that best musicians actually worked less than the average ones. But, the difference was that the best had 2 highly focused 1.5-2hrs practice sessions per day (deliberate practice) and rest for the remaining of the day, while average had several shorter practice sessions throughout the day.
