Is B2 or C1 considered fluent?

Is B2 or C1 considered fluent? Both, loosely, because "fluent" is a folk word, not an official CEFR level. The framework describes what you can do, not who counts as fluent. Most people mean B2 when they say fluent: at B2 you can follow conversations and hold your own with native speakers without much strain, even if you still hit gaps. C1 is a step beyond, where you express yourself fluently and spontaneously and handle demanding work and study. If your goal is living your daily life in the language, B2 is usually where it stops feeling like work.

What the levels actually describe, in the Council of Europe's own wording: B2 is an "independent user" who can interact with native speakers with enough fluency and spontaneity that regular conversation works without strain on either side. C1 is a "proficient user" who uses the language flexibly for social, academic and professional purposes and picks up implied meaning. Neither is a hard line, and you'll feel fluent in some situations before others. If you're weighing what fluency costs in vocabulary, that's a separate question: see how many words a C1 speaker knows.

Source: Council of Europe, CEFR level descriptions.