The Common European Framework for Reference of languages (CEFR), appears to cover the most frequent 5,000 words at the C2 level, which is the top level of the test. You can check here on page 186. To pass C2 in English, one needs a vocabulary of around 4,000 or more words. For French, the learner should know 3,300 - 3,700 words to pass C2. These figures are a far cry from what the native speaker knows but apparently enough to perform well on the most advanced level of the CEFR test. So I wouldn't quite call C2 mastery of the language, nor equivalent to a native speaker.
However, the description of C2, if true, does sound like someone fluent in the language:
Can understand with ease virtually everything heard or read. Can summarize information from different spoken and written sources, reconstructing arguments and accounts in a coherent presentation. Can express him/herself spontaneously, very fluently and precisely, differentiating finer shades of meaning even in the most complex situations.So, if I were planning to learn a European language and wanted to know what kind of target I should set for the number of words that I needed to learn within my time frame, I think I would go for about 5,000 words. If I were learning French, I would expect to feel pretty advanced by the time I reached the 3,500 known-words mark. If I didn't feel so, I would start to wonder what was wrong.
Because I don't need to set a deadline, I would calculate my arrival at "advanced level" by going from my target words per day and figuring out how many days that should take me. I think I wouldn't learn more than 5 words a day, so it would probably take me 2 years to learn French. However, I already have studied French so I wouldn't be starting out as a complete beginner.
In reference to the CEFR level descriptions, my Japanese is around a B2 level, in case you were wondering. I passed the Japanese Language Proficiency Test level 2, five years ago. I wish I had been learning 5 words a day since then.