Panko is made from bread and is susceptible to mold, especially if you are using fresh panko that hasn't been dried in the oven. Fresh panko should be used within a few days. If you dry your panko in the oven, it will last longer, however I recommend storing it in the refrigerator and using it within a few weeks. Store bought panko should be stored in the refrigerator once opened, and used by the expiration date.