Your 2 years estimate is way too conservative, I've seen people do it in a year, sometimes a few months if the languages are close enough (French-Italian-Spanish-Portuguese etc.)

Then there are all the really intensive courses/immersion programs where failure is basically forbidden (FSI, DLI, French Foreign Legion) and people reach B2-C1 very quickly, even for 'hard' languages (like Japanese or Arabic for a native English speaker, or French for a native Chinese speaker).