Most of the food (canned, packaged, whatever) has an expiration date on it. My recollection is that canned (not foil-packed) freeze-dried food has the longest shelf life.

The problem is, you really need to pay attention and rotate it out of storage and into consumption. You don't want to buy a bunch of 5-year shelf-life food and have the aliens hit 4 years later. You've only got a year's shelf-life left.