I don’t think you would ever stop learning your target language despite you being apparently fluent,
you can always work on pronunciation and be able to speak without any kind of foreign accent. That you would be technically proficient. And that can take much longer but it depends on the individual.
But I’m sure you will reach a level of satisfaction to be able speaking your target language without thinking. That it’s imprinted into your subconscious
Oh I see. Well, yes, they're different but sometimes it's just hard for me to switch between them. Like when I'm watching a Chinese cartoon and then my Japanese language partner sends me a message and I want to reply but my brain is giving me Chinese sentences instead of Japanese ones...
Yeah, I don't really mix then up. The knowledge of several languages helps me to learn others better. I have a hard time speaking Italian though because my Spanish interferes but Chinese and Japanese are different enough.