How long does the brain store unnecessary information?

I was an A-grade French student at school, but seven years after I graduated from high school, I was "horrified" to find that I had completely forgotten the language and could only remember what the first-graders were taught in the first ten classes, and about sixty words. Obviously, in all these seven years, I almost never ran away with my knowledge of French and studied another language. But I still wonder if this is an ordinary situation. How long should the average brain store unused information?