How long are licensees required to maintain ATF Forms 4473?
Licensees shall retain each ATF Form 4473 for a period of not less than 20 years after the date of sale or disposition. Where a licensee has initiated a National Instant Background Check System (NICS) check for a proposed firearms transaction, but the sale, delivery, or transfer of the firearm is not made, the licensee shall record any transaction number on the Form 4473, and retain the Form 4473 for a period of not less than 5 years after the date of the NICS inquiry.
https://www.atf.gov/firearms/qa/how-long-are-licensees-required-maintain-atf-forms-4473