There's a big difference between learning a language to a useful level and learning like a native speaker.
I am fortunate to live in a multilingual country, and it is obvious when experts and politicians are speaking in their mother tongue[1] versus in one of the other national languages or even in english. However, we judge them by what they have to say and how they express it[2], not by their accents or minor solecisms.
For what it's worth, the US State Department has ranked their experiences with teaching languages to anglophones going on foreign assignment, which corroborates my (and my friends' and colleagues') experience of closer to 2 years than to 30 for an 80-20 knowledge:
https://news.ycombinator.com/item?id=24677256
[1] which is not necessarily what the academies and middlebrows of other countries would consider the "correct" form of "their" language https://news.ycombinator.com/item?id=24351229
[2] Somewhere I heard the joke that french politics is full of dull people attempting to sound intellectual, but english politics is full of educated people attempting to sound common. Our politics is closer to the former than the latter. I just heard an interview with a train driver on the radio, and although her vocabulary revealed frequent repetition, she did her best to speak with a "langage soutenu" (a refined style, as opposed to the "langage de la rue", of the streets).