How long will it take to become fluent in French?

If you only count on the french class you'll have in school, definitely not. I'm french, I've been learning english for eight years, and I'm not fluent. Last year, I started learning english by myself, at home and my english improved 10 times faster than in school. I am now able to speak in english without even thinking about it. You will have to make efforts in order to become fluent. French is a difficult language, and it will take time, but eventually you will achieve your goal. My advice is: learn the basics, then as soon as you feel comfortable, watch french movies, listen to french music, change your computer/phone language settings to french, speak with french people (internet is amazing if you wanna meet foreign people), do anything you can in french. Never forget that a language has to be spoken. Theory is great, practice is better. Good luck! :)

It depends how much you work on learning French. I very much doubt that if you just take regular classes for four years you will be able to speak fluently. I have been studying German for two years and I'm far from speaking fluently. You would have to devote large amounts of time to learning it in order to acquire the language in four years.