With respect to medical insurance claims, a 3-year look-back window means that one can submit claims to the insurance company for events that occurred up to 3 years earlier. Therefore, I suppose that in marketing promotions too a look-back window is the period during one can still take advantage of the promotion. The product with the longer look-back window is likely to bring in more customers. It is a sort of deadline, but looking back in time. I am guessing a bit regarding its applicability to your specific scenario but I hope that it helps.
I am not sure if they are talking about an edible cookie or a web-related tracking cookie!