Fluency is when you understand at least
98% of your immersion.
For example, a typical episode of anime contains 300-400 lines of dialogue. If you watch the episode and encounter only 7-8 or less unknown words, you can call yourself fluent.
Your level of comprehension depends on the domain of interest. If you are not a mathematician, watching a lecture on mathematics may result in a lower percentage of understanding of the material, until you learn what you don't know. But, if you have a near-perfect comprehension of everyday speech, anime and dramas, comic books, then that is considered fluent.
In other words, fluency means being comfortable living your life completely immersed in another language.