There's the Survival Rule of Three which is that you can hang on for "3 minutes without air, 3 days without water, and 3 weeks without food." And then there's the approach that in all things survival, you need a layer of three, including food storage.
For example, Selco wrote an article a while back about layers when it came to bugging out. Basically, you need a layer close to you (as in on your person), a layer for more intense situations within easy reach, and another one someplace in your bag.
The same is true with food. Every prepared family should have multiple layers in their food storage. Let's take a look at the three layers of food storage. (Don't forget to take your family's dietary restrictions into account when building your supply.)
Layer 1: Stuff with a shorter expiration date that you'd use if you can't get to the store for a few weeks
Layer 2: Stuff that will last a year or so that you'll use during power outages or longer-term emergencies