Most cake mixes have a shelf life of 9-12 months. This does not mean that you will die if you bake a cake from a mix that is older than that. It just means that the quality might not be as good. How long past an expiration date you can use a box cake mix depends on how it was stored. If it was in a cool dry place then it could be good for 18 months - 2 years. If it was in a non-temperature controlled environment or bugs had access to it then I would throw it out.