I've been living in a hist family at age of 16, going to school there as well. I roughly knew the language before and I'd say I was pretty fluent after 3 months. I think it is a great thing to do as long as you are a pupil. I can imagine as a student things might go well, too. You get to see quite a lot people and students are still open for new social contacts so you will make friends and can practice the language. As an employee I see things differently. You don't meet as many people, you do not have as much spare time. Any grown-up foreigner I meet has a harder time meeting natives and making friends, and thus learning the language. The foreigners somehow stay in their groups and do not mix as well as the young people do.