According to Painters USA, opened paint can last more than two years if it is properly sealed and stored in ideal conditions. If the can is unopened, the paint can last up to 10 years, depending on the type and manufacturer of the product.
Paint lasts longer if it is stored in a cool place and no air is allowed to get into the container. To determine whether or not leftover or unopened paint has expired, check the consistency of the paint. If it is lumpy, stringy or dried out, it is not usable. If the paint does not blend well when stirred, it is expired.