I'm not chary or Weby, but I can answer how I feel about fluency. I speak English, Brasilian Portuguese and French. I studied French for 5 years at school and off and on for 3 years at uni, with the odd refresher course and trip to France since then. I have a very high level of French proficiency in reading, writing, listening and speaking. I can be dropped in the middle of France where nobody speaks English and get on perfectly well, make jokes, ask questions, hold conversations. All the same, I do not consider myself fluent. For one thing, I have trouble understanding people who speak very quickly, on telly and sometimes one the phone, or with strong regional accents sometimes. I also feel that my vocabulary is too limited to be considered fluent. I usually get compliments from French people on how well I speak and how good my accent is, but they still know straight away that I'm not a native speaker. I just don't feel fluent.
With Portuguese, I grew up speaking with my family, and spending June through August in my hometown in Brasil for most of my childhood. I never went to school there, and I formally studied Portuguese for all of 2 semesters at uni just to feel that I'd learned some formal grammar. My Portuguese is almost entirely casual and conversational, and compared to my English, my Portuguese vocabulary is very limited, probably comparable to a young teenager. However, I consider myself fluent in Portuguese, because I understand it with total ease in all situations (ok, I have some difficulty with very strong Portugal accents, but so do most Brasilians!), can learn and retain vocabulary and come out of being rusty almost instantly, and even have an easily identifiable (to other Brasilians) regional accent. It's technically my native language, as it was the first one I spoke, and I feel the same fluency in it as I do in English, just limited in my practice of it.