I think that it would be more appropriate to state it as the 10 year rule? After all who would be able to count all the hours.
No one seems to have quite got this 10 year/10,000 hours idea right so far. There's an additional factor: the 10k hours aren't just hours spent practicing, they have to be hours spend pushing the envelope, going where the practioner hasn't been before.
One of the better illustrations I've seen of this is the example of amateur golfers. It is apparently quite common for keen, determined amateur golfers to quickly rise to a decent standard then get stuck. The obvious explanation (normally assumed) is that they improve to their natural limit. The 10,000 hr proponent's interpretation is quite different and goes like this.
At the beginning, when motivation is very high, the newbie golfers practise intensely despite the obvious difficulties of starting a new skill. They improve rapidly at first. Then, when they reach a certain level of proficiency, they stop improving, quite suddenly. However, it's not that they stop practising. No, they keep that up, and likely still feel they are working hard to improve but they make no further progress. At this point is quite normal to get frustrated and blame the lack of development on the limits of talent.
What the 10,000 framework says is what has happened is a subtle change in their practice regime. Instead of constantly pushing beyond their skill level, they start repeating the things they can already do. And that is apparently the key factor of the idea: it's not practice as such that is important, but practising what you can't currently do...
The reason this happens to people is primarily motivation limits not talent limits. Because they reach a level of skill where they can play well and have enjoyable competitive matches sub consciously they relax because they are satisfied with where they are. On the conscious level, they still recognise that practice is necessary but because constantly pushing the envelope is mentally and emotionally demanding the young golfers take the easier route of practising what they can already do instead of slowly learning new skills. And that is why they stop developing.
With the photographic field, the idea there is "talent" that cannot be achieved by practice translates into "a skill that can be achieved but only by people who are prepared to put the effort into working on things they can't currently do, inch by inch, remorselessly for a very long time...".
That is not to denigrate the existence of raw, precocious talent, it is certainly around. However, it is quite rare that precocious talent translates into world class life success, as many long term studies of "genius level IQs" have shown. People tend to romantically imagine there is no substitute for natural talent but the real world usually shows that there is really no substitute for obsessive determination...
EDIT: Sorry, should have read ALL the posts before pontificating. Gladwell's ideas are obviously reporting prior research but yes the term I was looking for was "deliberate" practice.