https://preply.com/en/blog/english-language-levels/
C1 is an advanced English level. C1 users can speak English with ease, and understand the language in (almost!) all of its complexity. By this point, you will be able to have longer conversations, even about unfamiliar topics. You’ll also comprehend longer English texts. At this stage, you can use English day-to-day for business and academic purposes. If B2 is what many consider “fluent,” then C1 is fluency with increased nuance and understanding. At C1, you can understand subtle jokes in the language, and express yourself with colorful native phrases.
When you reach C1, you should have a working vocabulary of about 8000 words – almost double that of B2! It takes approximately 700-800 hours with the language to pass the C1 Cambridge examination.