Asklemmy

For me personally, when you reach a level where you can think, and communicate in the non-native language (without doing mental translations back and forth) with enough ease and speed, no mater the topic at hand (meaning that even if you don't know a technical or specific word you can make yourself understood), and even if you make grammatical mistakes or have an accent, the point of the conversation is not lost between participants, then you can consider yourself fluent enough on said language.

My native tongue is Spanish (could you tell if I didn't mention it?), but I have consumed so much content throughout (and yes I did check how to spell throughout) my life only in English and practiced enough doing conversations both writing and speaking (even with an accent) on the internet that I can communicate with ease and be understood.

I have visited the United States a handful of times for around a month for vacations with family, so I can say that I had to communicate with native people outside the internet now, but I haven't had any formal education except a few very basic English courses in high school.