The section from Gladwell's Outliers -- positing that one requires 10,000 hours of practice before one can be considered an "expert" -- came up in the writers' room recently. Almost habitually, I did the Fermi Problem in my head as I was leaving the parking lot Friday.
On a good day, I get a solid four hours of productive screenwriting done. That's actual head-down typing, new words produced, problems solved with pens and flowcharts on card-stock. For every weekend I've plowed straight through I've utterly goofed off, so lets say 5 days therefore 20 hours a week, so 5 weeks gives me 100 hours let's multiply by 10 to get 50 weeks a year (vacation for 2 weeks) producing 1000 hours per year ... wrote my spec feature in '99 -- I'll just cross the threshold this year. For television writing you could argue 12 years (12,000 hours) but I had a whackload of years off in the middle there where I just wrote pilots and never staffed. You could argue I've got no more than half that time elbow-deep in the bloody guts of broadcast television writing and producing.
To tell you the truth, I'm not sure what that means, other than a.) put your head down and take every job you can, and assume b.) you'll suck for quite a bit of it and c.) ultimately it won't matter as long as you do a.) and build your toolbox.
One might also assume that my actual expertise -- gleaned from doing stand-up since I was 22 -- is in convincing strangers that I am amusing and competent, and assume that expertise over-rode my lack of competency in sheer craft.
Regardless, this only supports the idea that any advice I offer about writing on this site should be taken with a rather large grain of salt.
Submissions, please, for your Odd Area of Expertise, in the Comments.
(Oh, and the first person with any variant of "well, considering the quality of your work I'm not surprised" Comment will be banned simply for screaming unoriginality. I mean, really. Stretch a little.)