If stored properly, the shelf life of cake mix is four to five months after its "best before" date. How long a mix lasts before expiring depends highly on how it is stored.
The date printed on the box is the best-by date and not an expiration date. Cake mix that is stored unopened in a cool, dry environment does not need to be used by its best before date. However, if the mix is stored in a humid environment, at high altitudes, near heat sources or accidentally gets wet, it should be used promptly and prior to the printed "best before" date on the package.