1. Pig feed as a shelf life, typically about three months. After that the vitamin content starts to go downhill - it does not fall off a cliff, but just be aware.
2. You'll need to store it where it is vermin proof, otherwise you'll attract rates and mice, so consider this.
3. You can get discounts in some places for buying say 5 bags at a time, so you may be able to save some money that way.
4. Your "local" store may not be so local, and you'd spend more on fuel if you bought each month.
There is no right answer, you'll need to think about the above for your circumstances