As far as learning a language
goes, it's hard to figure out exactly when you become fluent. Webster1913 tells us that you have to be "having words at command; and uttering them with facility and smoothness." Here are some ways to gauge your fluency in a language:
Subliminal Translation - If you have to translate the language to your mother tongue in order to carry on a conversation, you're probably not fluent. Fluency entails being able to think, at least on a basic level, in the language. If someone screams "Fuego!," and you don't haul ass right away, then your radical ideas about fluency are pretty much shot to hell.
Wacky Foreign Dreams - If you start having dreams in a foreign language, however sporadic, you're at least approaching fluency in that language. Again, it's a question of whether your subliminal thought processes can function in that language. If your brain can dream in the language, it's likely that the language is at least partially hard-wired into your automatic thought processes.
Accidentally Using The Language's Words In Your Mother Language - I once caught myself saying "sumimasen" while pushing through a crowd at Aventura Mall, and that was one of the happiest fucking days of my life. But on a more serious note, if foreign interjections begin to seep into your native tongue, you're probably in the early stages of fluency. If you do this intentionally, on the other hand, you're a dork.
That said, there are also some criteria that don't have anything to do with fluency. First off, your accent is unrelated to fluency: even if your tones sound wacky when you speak, you can still be fluent in Chinese. Vocabulary beyond a basic level doesn't impact fluency: you can memorize a French dictionary, but if you can't recall the words quickly and naturally in conversation, you aren't fluent in French. (If you want to become fluent in a language quickly, in fact, it's best to stock up on very general words. Even if you can't remember the word for "microchip," you can cover by talking about "that little thing on the green card thing in the inside of the computer.")
It's not too difficult to be fluent in a language, but it's much harder to be really good at it. Consider all the native English speakers who can't distinguish between "there," "they're," and "their." Then pop open your textbook and relax.