I live in the United States where I would ask immigrants often how long it took them to get that good at English (~C1)
My conclusion is ~5years and no more than 10yrs
I think you can use this benchmark with most languages and reach comfort in knowing that if you’re fluent in any less than 5 years, you’re a badass
EDIT: would like to add a few notes about, for example, famous YouTubers
AJATT Matt smashed Japanese for 5 years and Metatron needed 8 years for Japanese
Xiaoma required 3 years for Mandarin and he was in China for like 2 of those years
Norwegian With Ilys smashed it for 2 years up to ~C1
A seasoned language learner should be able to reach their goals with a language in under 5 years unless your language is really really hard. I am 18 months with Bahasa Indonesia and I think it’ll take another 18 months but I’ll let you know!