It doesn't have an expiration date.Unlike other things you get from the supermarket, you can't see a quality guarantee period printed on the package. You can use it as long as it is not broken or scratched.
If the sunglasses have a good UV protection, then it will be valid until your sunglasses is broken or be damaged. If the quality of UV protection is not so good, it may lose its effect with prolonged use.
So far, i didn't heard that UV protection of the sunglasses have a expiration date. Usually, people buy new sunglasses are because of tiredness of their old sunglasses. Or the old sunglasses are broken that can stay on their face. But i never heard that people meet expired sunglasses unless the sunglasses have no UV coating on the lenses.