Estimating that one chin eats about 2-3 pounds of food per month, that would last about 4-5 months. I do have a concern though - buying a bag that large and having it for so long can pose potential problems. A lot of owners here (with a small number of chinchillas) buy food a couple pounds at a time from various members here - this guarantees that the feed is fresh. I personally have had bug problems with old feed, and there is speculation that the older a feed is, the less nutritional value it has.
Just some food for thought. You can buy feed from a lot of places, I personally like QMC, but you can also buy from Camphor Chins, and there are lots more that I can't think of right now.
Just put yourself in your chinnie's fur - would you want to eat 5 month old food? Gross!
Store the pellets in an airtight container. They should be good for about 6 months after the mill date on the bag. After that they will start to lose nutritional value. That is the rule that I always heard.