Cake mix is safe to eat for four to five months after the best-by date printed on the package. After the best-by date, the quality and flavor of the cake mix may degrade according to Duncan Hines, and the cake may not rise properly.
To extend the life of a cake mix, keep it in a cool, dry place. Once it has been opened, the package should be kept tightly sealed to prevent moisture and contaminants from getting inside. If the cake mix has developed an unusual odor, a difference in appearance or flavor, or mold, then it should definitely be discarded and not eaten.