The only thing you can say for certain is that if someone is to be called “fluent” then their speech and/or writing should be relatively smooth, it shouldn’t seemed forced, choppy, or difficult for them to perform, and even then I guarantee you I’m going to get shit over that definition (someone will pop in and say how someone can still be “fluent” while having trouble speaking in the language in question, just watch it’ll happen). Just to back myself up again, from the Wikipedia entry for “fluency”:

Language fluency is used informally to denote broadly a high level of language proficiency, most typically foreign language or another learned language, and more narrowly to denote fluid language use, as opposed to slow, halting use.

Look, that’s pretty much it as far as a concrete definition is concerned: it should “sound smooth”.

What do I consider fluent?

I personally have two different types of fluency, I think this works really, really well, and it’s very simple and easy to understand:

1. Native fluency: Simple, this is when you’re at the same level of proficiency in the language as a native speaker–you have their vocabulary, you know all the idioms and slang and cultural references a native would, and you have a perfect accent. If a native speaker were to have a 3-hour-long conversation with you face-to-face (presuming your appearance wouldn’t give you away as a non-native) and they were asked afterward by someone if you were a native speaker of their language, they would say with 100% certainty that you were.

I’ve known one person like this, a Japanese girl I knew in college named Saria. She came to the U.S. for the first time when she showed up for college (she would’ve been 18), and she spoke perfect American English, if you had talked to her for an hour or so you would’ve sworn she was an American (obviously of Asian descent due to her appearance, but definitely American none the less). I seem to recall she had been learning English since grade school and had taken a personal interest in it and American culture, so she watched lots of American movies and learned the accent and all the slang and idioms and such from that.