The idea is to have “fresh” insulin. There are many types and kinds of insulin delivery systems. It is important to use a system that has an appropriate amount for the quantity you need. Insulin is expensive and you don’t want to have a quantity more than what you will need for a month at a time because you’ll end up having to waste it. The “month” thing is kind of an arbitrary time to help ensure freshness. It is a little like “expiration dates” or “use by dates” or “best by dates” on other products.