I'm not sure that question has any real meaning. I think most artists move from one goal to the next. There's always something out of reach to you currently that you aspire to. I wish I could paint seascapes, or I wish had the command of color that so-and-so has. I don't think there are any specific levels, because each artist brings a different set of skills to the table.

I think a better question would be, am I expressing what I want to express? If not, why not? If my dogs look like turtles, what can I do about it? Draw lots of dogs, train your eye to pick out the shapes of the dark values, watch other artists draw dogs and study what they are doing differently.

Once you master that, some other thing will start to bother you and call out for improvement. Or maybe you'll tire of graphite and decide to start putting some color in your work. It's always something, that's what creativity is all about.

If you ever get to the point where you feel you have reached that "magic level" where you can stop growing and changing, then the artist in you has died.

 

I see two questions in your post so I will comment on both.

Based solely on my personal experience, I think that I know the level I want to get to by looking at my art and acknowledging my feelings about it. If it is looking and feeling as I envisioned then I have reached that level. That doesn't mean I can't push forward. When I used to sketch/draw, my trouble was that I couldn't get past a certain level and found that disheartening. I eventually moved on from it to 3D modeling where I became very proficient and I could produce new works without having to think about my tools. I could just enjoy the process and was usually pleased with the results. With the drawing, it always seemed like so much effort every time and I did it for years and produced hundreds of drawings.

With regards to testing, I personally hate being tested or judged. In all my personal art projects I just create for myself, without worrying if others like it. In my work world when I created for clients I was able to take their direction and not take it personally, since they had their own vision of what they were after and I was just the tool, or the means to get there.

In the end, I think it is important to enjoy the process of creating art without living for the end result.

 

I hate to be the one to bring the bad news to you, but unless you are an AI (and the response then is "whenever users do not ask for improvements", as a human, you will never reach the level you want: you will strive, and as you get any close, you will gain confidence and then shift your expectations pushing them a bit farther, just before you reach your previously desired level.

That's only human nature: shifting expectations.

OTOH, if what you want to know is when is your art good-enough, the answer depends on additional qualification of the question: good enough for what? And the answer is 'as soon as you can see it accomplishes the goal you had in mind'. You can always get better, anything one can do can always be improved, but if it does its work, then it is good enough.

Problem with that answer is you may discover it does what you wanted well before it looks any good to you, but believe me, you will always see some way to improve and it will never look good enough, so you must learn to ease on yourself and accept the fact that even if it is good enough, you will always want more, and that that search for excellence is innate and inherent to all human beings, and that it is natural, and all good and well too.