TL;DR All Apple products carry a standard 1-year limited warranty, including accessories.

However, Apple accessories that are bought for and used with products covered under AppleCare extended warranty (like Mac or overseas iPhones) are also covered under that extended warranty. But if you buy it during the year, that power adapter is warranted for the full 1 year, even when your MacBook extended warranty has ended.

E.g. you have a MacBook covered under the 3-year warranty. If you buy an extra power adapter during your first year, that power adapter is warranted for as long as that MacBook warranty stands.

P.S. this also works with countries that require companies to have more than 1-year warranty (like UK or Australia)