From what I gather, the cashback comes in the form of a statement credit (ie onto your credit card account) each month - but as I don't actually have an account or card with PenFed, I could be wrong. Even if I'm right, there may be an option to get the cashback another way, I don't know.
So... not much help, then - sorry!
Since we use the card for all our gas and grocs purchases, I don't know that we've ever gone a week, much less a month, without using the card.
You might need to call CS on this one.