DEET, the active ingredient in bug sprays, does not lose effectiveness. Bug sprays have no expiration date, but they do have a shelf life of three years. A product's shelf life is typically a period of time in which the product performs at its best, according to manufacturer standards.
According to DEET Online, the ingredient itself does not lose effectiveness over time, but manufacturers add other compounds to repellent bug sprays to enhance its scent, feel and appearance. The Food and Drug Administration does not require repellent companies that use DEET as its active ingredient to state an expiration date as it is highly stable and effective indefinitely.