To get "Good" is a really tough thing to answer. I think I am really good at brewing one beer. My amber. I have the recipe down pat and you cannot tell one batch from another. I have swapped some ingredients out before, using different crystal malts and have swapped out hops and only then could I tell there was a difference in the beer.
My other beers I think are still pretty good as I seem to have my system tuned pretty well, but basically, to use the chef analogy from above, I'd consider myself a good cook right now. I have the tools, know how to use them, can repeat a recipe pretty well, but as far as being innovative, really being able to predict what results may come from tweaks, or really stepping out and designing quality beers every time out, I'm still a ways off from that.
I'm pretty lucky in that things have always fallen into place pretty easily for me. My mash is almost always within a good ph range without doing anything to it. My water quality is good once I get out the chlorine. My basement temps usually fall in a range that temperature control for my fermentation is no more than a towel and an ice pack. And I've always been a troubleshooter and tinkerer since I was a little kid. So if something seems a little off, I'm pretty good at breaking things down and finding the issue.
As far as truly "Good" goes though, I'll probably have no idea beyond what my friends say and what my taste buds say. I really have no urge to join a competition, so I doubt I'll ever have any proof saying weather or not my brews are good. I know my friends call me to come over to drink my beer and always ask me to bring it to parties. I really enjoy what I brew, so, that's good enough for me.
In the end though, proper fermentation, imo, is what really separates good beer from just beer. I'm obviously not the first one here to say it, but pitching the right amount of yeast at the right temperature really does a lot towards making good beer. As I said above, maybe I'm lucky in how everything comes together and I've always fancied myself a decent cook, so brewing is probably just an extension of that. I would think things would really have to go awry to not produce a wort, that with proper fermentation, wouldn't be decent.
I don't think it takes that long to become a good brewer. Following basic practices, learning your system and proper fermentation are all you really need to produce good beer. I think the tough step is moving to that next level. Producing great beer, that's the trick. Maybe it's just because there are so many mediocre commercial brews on the market, that I don't think there is anything that hard to producing a decent tasting, drinkable beer. The key is being able to produce that beer every time you brew, no matter what style you brew. That just takes years and years of practice. Like anything, you don't become good by reading about how to become good, you do it with practice.