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.