is it possible to pass Topik6 in 3 mouths from 0?

Absolutely impossible.

The best I've ever heard of was a Chinese guy (also fluent in Japanese, which helped) and an absolute phenom in languages. Like just extraordinarily, superhumanly talented in languages. He studied 14 hours a day (roughly 100 hours a week) and managed to do it in about 18 months, if I remember right.

For people who have only been studying/practising for 40-60 hours a week, then somewhere around 8-10 years is more realistic. 5 or 6 years may be plausible if you push yourself beyond the 60 hours a week.

Don't underestimate how difficult level 6 is. Even for native Koreans, who have spoken/written the language their entire lives and are forced to study advanced vocabulary and grammar formally in high school, many would not be able to reach TOPIK level 6.

/r/Korean Thread