When do you consider yourself to be "done"?

[anna3101]

5 hours ago, Blaveloper said:

It sure is possible, that's what we call "C2 level".

Mastering all the nuances and expressions: I'm not sure what you mean by this one.
Mastering accent: Olly Richards always aims on the accent from the very beginning. He has a nice Egyptian accent while speaking Arabic, while he still has his British accent while speaking English for example.
Being able to read and write perfectly: English is not my native language, but I can read and write it much better than most of the Americans who don't even know how to distinguish "your" from "you're" (annoyance #1).
Mastering sentence structure: I don't understand what's so hard about this one. Though it can be hard in languages with lots of exceptions, but if you use the language non-stop every day, it'll all become natural to you at some point.

C2 level is not native, but "near native" fluency, and that's not at all the same thing. Personally, I agree with Chris_A - I don't believe it's possible to completely master a foreign language, and even if it is, it probably takes a lifetime.

I've met some people who were extremely talented and had great pronunciation and grammar. But there will inevitably be a slip here and there from time to time. A wrong accent, or a small grammar mistake. And even if you are a native speaker who had decent education, no, I don't think that means you've mastered your native language. Everybody makes mistakes and everybody will sooner or later come across words they don't know. For as long people are not born perfect, learning ever ends.

[宇崎ちゃん]

I'm aware of that too.
But "mastering" and "being perfect" are 2 different things.
To me "mastering" means "being fluent enough to heavily use the language", which doesn't take me a lifetime at all.
I can understand it takes a lifetime for most people, but that's because they either learn it the wrong way, or they give up at an early or intermediate stage.

Making mistakes is actually a good thing when learning a language (or rather, when learning anything at all).
If you know you're doing it wrong, your brain will do its best to avoid doing it again.