I don't have any experience (first or second-hand) of going from complete ignorance of a language to complete mastery via an intensive language course.

However, I know someone who had taken a couple of mediocre college-level courses in French, then went to a French language school for 4-6 weeks (he was tested and enrolled in an intermediate-level course). He emerged with quite a good mastery/fluency in the language. Of course, it was a total emersion experience in class and out (he boarded with a French family).