I started applying this to random things, and I think one could cut it down to even 10 hours for things one would just want to have a familiarity with. It's surprising how much progress can you make in few initial hours of practice. I could never toss a coin without it landing in completely random places or hitting someone. After just 3-5 hours of practice total over span of few days I learned how to toss various coins and catch them with one (the same) hand, and I could even spin them in ways that make or don't make sound on demand.
So my take is: apply 10-hour rule to random stuff you fancy, things that could make good party tricks, etc. Apply 100-hour rule to things you end up caring about enough that you want to be proficient in them.
* 10 hours: familiar
* 100 hours: proficient
* 1000 hours: good
* 10000 hours: expert
1 hour = familiar (heard of it)
10 hours = proficient (1 day)
100 Hours = "good" (2 weeks)
1000 Hours = "expert" (6 months)
Although a general rule of thumb (that is true) is 3 months of full time work on a topic makes you "conversant"--good enough to pass yourself off as an expert to the unsuspecting. OF course, YMMV.
A while back I started learning the guitar - having learnt drums and (some) piano as a child with a teacher and structured learning, I decided to teach myself at a much slower process for 'fun'. I'm probably somewhere between the proficient and good categories now and I think this trajectory matches my experiences.
What is an important take away for people is that you can learn something, and it doesn't take that long for you to get to a point where it is useful, enjoyable and satisfying. So many adults shy away from learning new skills unnecessarily. Learn to play guitar. Learn to code. Learn to paint, learn to speak Dutch, learn to write poetry, learn to bake, learn to woodwork. Your life will be richer for it.
Moreover, I've found these minor skills have given me broad applications beyond what I originally intended (leisure). I worked at a day camp and knowing the harmonica and ukulele made campfire stories/skits/songs immeasurably better (I also did a semester of intense practice in performance literature, which helped as well). My forays into design have made me the resident web/design expert in my mathematics department, resulting in some extra cash. Learning to paint has made decorating my apartment way more fun and elevates my perceived sophistication (which I think is hilarious). The list goes on and I don't intend to stop growing it.
Thanks for validating the concept!
Of course, if you keep going, you get the first 800 hours worth of skill in less than a second (and the first 300 in a mere millisecond), so I'm not sure the repeated application is justified.
The question is WHICH of those 80 hours account for 51.2% of the value of 10K hours of practice. It's not necessarily the FIRST 80 hours.
It's very possible you could get 8% of your skill in less than a second; it just might come at a lightning-strike eureka or moment of epiphany during hour 8,679.
Probably not. I don't think that whoever came up with 80/20 had this in mind, but that's the fun with mathy-sounding rules of thumb - you get to mix them and/or fold them on themselves and see what comes out ;).
More seriously, you may try to rescue the rule and such application by noting that it is the right 20% of work required to get 80% of result. Finding that work in the space of all work to be done requires effort on itself. Finding the right 4%, or right 0.8%, requires even more bits of knowledge, thus more meta-work.
Fortunately, we may cheat a little bit, because those who came before us and mastered the skill we want often already did the big part of figuring out which work has most payoff at the beginning.
 - http://en.wikipedia.org/wiki/Pareto_principle.