The concept of 10,000 hours to becoming an expert is often very misunderstood. Many believe that simply practicing all those hours automatically qualifies one as an expert. This concept first was discovered by Herbert Simon, who found that experts learned approximately 50,000 chunks of information usually over a 10 year span. Anders Ericsson also researched into this concept and discovered the importance of deliberate practice.
Deliberate practice is "not inherently enjoyable," according the Ericsson. It requires working in a constant struggle, pushing limits, and reaching just outside your grasp.
Simon included a key word that many will over look in his definition of an expert, learned. Those who consistently push and learn are those who become experts. Simply going through motions for 10,000 hours and staying in a comfort zone does not make one an expert because they have 10-15 years experience. What's the quality of that experience? Did they learn 50,000 new chunks of information, or did they just observe?
A great example in my own life was when I was in high school and used to shoot baskets for hours upon hours. I would shoot jumper after jumper but often it was a set shot, without dribbling or a casual pull up, over and over. As a result I was a good shooter when open, but off the dribble or when guarded closely, I would struggle. If I had added moves, pushed the speed of my release, or even changed to different spots (free throw to baseline perhaps), that would have been deliberate practice. Shooting the same shot over and over had a low transfer of learning to my actual game. I may have had as many hours as a lot of college players by the time I was 16 but I didn't have the same amount of quality work of someone who practiced more efficiently.
The old saying of it's not how long you practice but how you practice, actually is being scientifically proven by Ericsson and many others. The only difference they find after a certain point of expertise is the number of hours in deliberate practice.
I get worried when parents and coaches read about the 10,000 hour rule and think that putting their kid into 100 games a year and 5 summer camps that will speed up the process. The 10,000 hour rule is a long term approach that relies more on consistent and quality practice. Simply forcing kids into more games and specialization at a young age is detrimental is so many ways.
To sum it all up, 10,000 hours of practice depends on the quality of those hours. True experts seek consistent improvement of their craft, while the others stay content.