When I got my braces off back in high school (about 15 years ago now), I was told the same thing. I think I only wore my retainer for about 6 months until I got tired of it, then I stopped. My teeth moved a little, but not enough to make any significant difference. And over the years, my teeth haven't moved back to how they were originally. I think if you wear it for at least a few months, to a year, you should be fine.