Licensees must keep each ATF Form 4473 for which a NICS check has been initiated, regardless of whether the firearm was transferred.
If the transfer is made, the FFL must keep the Form 4473 for 20 years after the date of the sale or disposition.
If the transfer is not made, the FFL must keep the Form 4473 for 5 years after the date of the NICS inquiry. Forms 4473 for transfers that did not take place must be maintained separately.
[27 CFR 478.129(b)]