How long a money order is valid depends on the financial service provider from which it is purchased, as well as any applicable state laws. For example, Western Union money orders don't have an expiration date. Money orders purchased from the U.S. Postal Service also don't expire.
However, there may be state laws regarding how long a consumer has to cash a money order. Some states require that a money order be cashed within one year of the purchase date. Other states allow consumers to hold the money order for three years. Money orders also may be replaceable should they be lost or stolen, but this depends on the policies of the financial service provider that sold it.