27 CFR § 447.43 - Terms of permit.
(a) Import permits issued under this subpart are valid for two years from their issuance date unless a different period of validity is stated thereon. They are not transferable.
(b) If shipment cannot be completed during the period of validity of the permit, another application must be submitted for permit to cover the unshipped balance. Such an application shall make reference to the previous permit and may include materials in addition to the unshipped balance.