I haven’t spoken French very much since I was in my teens, but I’m still fluent and sound like a native speaker (although in a way that would give the Académie Française fits). My hometown in Ontario is 3/4 Francophone, and I attended a French (not immersion) school until I was 13, although my family background is Anglophone. I don’t recommend that as a strategy, though—I had a difficult time for the first couple of years.