In addition to the vitamins losing their potency, the proteins begin to break down. You're still getting calories, but you may not be getting the nutrition you need in the long haul. A bottle of multi-vitamins is cheap insurance along with rotating your stock. I also store and rotate a fair amount of fruit juice, which would hold out for a few months. Beyond that, I have a couple cans of tang and "emergen-C" packets. The problem with most vitamin C tablets is that air and moisture quickly start to break it down. As soon as you open the bottle, they start to degrade. On the other hand, if you've stored mung beans and alfalfa seeds, fresh sprouts are a great source of vitamin C and a tasty way to get away from packaged food, even in the dead of winter. I think buying a 25 pound bag of mung beans and breaking it down into 1 pint paint cans (with mylar bag liners and oxygen absorbers, of course) is a great way to ensure good year-round nutrient rich food.