If stored properly (away from moisture and allowed to breath), it can be good for over a year. Think that hay is harvested in the summer and has to last until the following summer. So even at the shortest time from the last harvest to the first of the next year, it is at least 8 months. I buy enough hay, by the bale, to last about a year. For me, that is about 3-4 bales.
If you have the space and money up front, I don't see why you should not by a larger amount of hay. You probably don't need more than 50 pounds at a time and should save money over buying the smaller bags. As you are probably getting another bun, I would buy hay in larger amounts. If they are not bonded right away (or you don't plan on bonding), then the amount of hay you use could double and that will get expensive.
I think (having not bought store hay in a couple years) that most hay bags are designed to store the hay, at least for a while. You could poke some holes in the bag so it can air our. I would not totally open the bag as it could brown the hay faster.