Here's the ATF's comment on how long to keep the records:
Q: Do FFLs have to keep a copy of ATF Form 4473 if the transaction is denied or for some other reason is not completed?
FFLs must keep a copy of each ATF Form 4473 for which a NICS check has been initiated, regardless of whether the transfer of the firearm was made. If the transfer is not made, the FFL must keep the Form 4473 for 5 years after the date of the NICS inquiry. If the transfer is made, the FFL must keep the Form 4473 for 20 years after the date of the sale or disposition. Forms 4473 with respect to a transfer that did not take place must be separately maintained.
[27 CFR 478.129(b)]
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