Here's a really unscientific explanation:
Corn and corn products tend to go rancid rather quickly because the middle part of the corn, the little seed part, contains oils that will become rancid over time. It isn't due to oxygen exposure as much as it is just a result of the oils breaking down. Cornmeal, for example, has the middle part removed to prevent that from happening. I don't believe they do that for flaked maize.
Cold will delay the process. I'd say open it up and smell it. If it smells ok, taste it. If it tastes ok, put it in your freezer until you can use it. If you don't want to put it in the freezer, at least put it in the fridge.