19 CFR § 10.137 - Records of use.
(a) Maintenance by importer. The importer shall maintain accurate and detailed records showing the use or other disposition of the imported merchandise. The burden shall be on the importer to keep records so that the claim of actual use can be readily established.
(b) Retention of records. The importer shall retain records of use or disposition for a period of 3 years from the date of liquidation of the entry.