It does somewhat depend on how fast your chin goes through the food and how easily you can get the food. We have 17+ chins, so we usually have 2 - 25 pound bags in our feed storage container (which we feed from daily) and then an extra bag on the side... but they also go through it really quick when you have this many.
With only one or two... we used to have a rescue on Oxbow and he'd go through one of the 5 pound bags maybe every month? As far as I know, food is good up to 6 months from the mill date, so depending when the mill date is, you could probably buy 2-3 bags and have them last you a few months...
I know there's a member on here who sells Oxbow, and if you're going to go with Oxbow, it'd be cheaper to buy from the member and get it shipped in than to buy from the store... I don't remember the membername, but maybe someone else can chime in. It's worth a thought if you're going to go with Oxbow, as it's one of the more pricy chin feeds.