I've been to France and I feel that it takes about 3 months to get used to the language everyone uses in everyday life. I was only there for 2 weeks so I don't know that much French other than High School French. However, my friend went in High school and is fluent in French and English. He went for 4-6 months at a time and went to school there. It gave him a huge jump from French III to AP French in High School. I only went up to French IV, it does help you in short term if you go for a few weeks, but 3 months will help you better in the long term.