(This was before the rule changed to 10,000 hours.) It could be interesting for someone more knowledgable about art to indicate at what point he hit 'mastery' (assuming that has already happened) and to then estimate how many hours it took him to get there.
In other words, every hour of deliberate practice brings the number down exactly one hour.
My reply to you is "yes." Additionally, I'd like to point out that indeed every hour of deliberate practice does reduce the amount suggested by one hour. Unless you have mastered manipulation of space-time, of course.
I think alnayyir is trying to say that deliberate practice can bring down the 10 year number (which I agree with and is why the rule switched from saying 10 years to saying 10,000 hours) and michael_dorfman is interpreting that statement to be referring to the 10,000 hours number.
My point in the OP was to show that by having a large amount of data, this provides a nice example for understanding why the rule was changed. (But for it to be a good example, it would require someone knowledgeable about art to look over his prior works and pinpoint when exactly he became a 'master' (although this may be a hard thing to do) and to then estimate how many hours of practice was spent to get there.)
Back on topic, the ten thousand hours is supposed to be deliberate practice to achieve mastery. One good source for this is Malcolm Gladwell's book, Outliers.