Having taught German for over 20 years, I have only seen very few occasions where someone would be able to learn C1 level German (the second highest level on the European Framework of Reference for Languages and the Goethe Institut) in one year, yet a significantly higher proportion of students who would be able to reach B2 level, which is upper-intermediate German. Having said that, I teach German in London and via Skype, so my students aren't immersed in the language in their day-to-day lives. The latter would obviously increase your chance of picking up the language more quickly, as your contact with the language would be more intense. But even so, the key is commitment to revision outside of lessons, a thorough grammar training in the lessons and (dare I say it) a good tutor. While the class room setting has its own advantages (such as interaction with peers), the intensity that is required to learn German in a year is best created in one-to-one tuition.
My final word of advice would be: be careful when choosing a German tutor. There are several teachers out there who think that German can be taught by virtue of what they call a "conversational approach". This is bogus. As I try to explain in one of my previous blog posts, grammar is very important in the process of learning German, more so than in many other European languages, such as Italian. And now have fun studying!
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