First of all, while I respect Gladwell as a dang good writer, and while I think Outliers was an interesting book, I think that 10,000 hours thing gets touted a bit too much by starry-eyed writers. You have to understand that 10,000 hours with specific feedback is a generality, arrived at by Gladwell through his own observations, which he then condensed into a digestible nugget of information as the basis for his book. That's all it is: one man's observation rendered into a sound bite. It is not a magic bullet, nor is it a rule.
Now, that being said, I think it's a pretty rational sound bite, all things considered. The more you practice at a skill with a specific goal in mind, and the more you seek critical feedback and use that feedback to improve your work, the better you will get at what you do. That's a given. What isn't often considered by so many writers on so many forums is that you can be really, really good well before you hit the 10K-hour mark. Many factors contribute to success. Practice and criticism are undoubtedly very important, but they will not solely determine whether you succeed. In my opinion, many more contributing factors come into play when we're talking about becoming a successful writer: observation, confident risk-taking, very wide reading (and reading critically, not purely for entertainment), deep empathy, the ability to be objective about one's own work, and more. To some extent, lots of these things can be picked up while you accumulate however many hours of practice. Others may be inborn characteristics that perhaps not everybody can develop, and perhaps others have already developed so well before they even think to try writing that their writing is welcomed by readers relatively early on. Who knows. All I'm saying is, there is not a magical formula. Neither X amount of time, nor X amount of practice, nor X amount of natural aptitude will make you a success. No one knows how all the pieces fall together to make a successful person a success, and a lot of people who follow all the apparent "rules" never get there, even though they do absolutely everything right.
So don't expect yourself to become a "good" writer after N years. Similarly, don't expect yourself, if you haven't put in your 10,000 hours of criticized work, to suddenly wake up a failure in N years, even though people told you you're a good writer.
The question here is, what is good? With a skill set like chess, you have a specific number you can put on "master," and you know exactly how many points you are away from that rating, and how many you need to accumulate to get you to your goal. There is no such cut-and-dried number for "good writing." To a very large and very real extent, you are good if somebody tells you you're good, assuming they are giving you their honest opinion. No book has yet managed to impress all its readers equally; no one agrees on who the best writer is. There are as many legitimate definitions of "good writer" as there are readers.
A singer may be said to be a bad singer if he can't stay on key. But the judge of a singing competition may also tell a singer he's not good enough to win simply because his style doesn't fit with the show's image. If he can hit all his notes and emote well, but he sings barbershop-quartet style instead of modern-pop style, is he a bad singer? Or is he a good singer? Depends who you ask, and under what circumstances, doesn't it?
You will never go wrong with your writing as long as you are always striving to improve. Work hard at developing objectivity so that you don't have to haunt your own dreams with questions of who is the chess master of writing. Because, dang, there are too many readers on the planet to count, and all of their opinions of your work may be equally valid or invalid, just like the various opinions about that barbershop-quarter singer, right? You should seek outside criticism, because it does make you better. I have had killer beta readers whose opinions and suggestions knocked my socks off and allowed me to see my writing in totally new and helpful ways. I have had beta readers who, I kid you not, told me I should look to Stephenie Meyer for an example of how to develop good characters. You may love Meyer's character work, but I find it absolutely abysmal. So some beta readers are great. Some are awful. It's not easy to find great ones. I've had great editors and terrible editors. I've had glowing reviews of my books and one-stars that ripped me a new one.
So the answer to your question is: there is no easy answer. If you find a beta reader who seems to get what you're trying to do and who makes suggestions that make sense to you, count yourself lucky. If you run into an editor who suggests changes that make you shake your head and wonder what they're smoking, don't be surprised. If readers tell you you're a good writer, then you are. If readers tell you you're a bad writer, then you are.
What do you tell yourself? And how did you arrive at that opinion? And what are you currently doing to actively improve your work? I think those are the most important questions to ask yourself.
tl;dr. Peace out.
