NFA application status depends on the type of transaction or ATF form that is used because each form has different requirements and different processes that require different processing times. NFA applications begin processing when the applicable form is entered into the National Firearms Registration Transfer Records database (NFRTR), at which point the application enters a “Pending” status until reviewed by an ATF Legal Instruments Examiner.
The time required to process an NFA application depends upon many factors including the particular ATF form used, the volume of forms submitted from throughout the country, any errors on forms, and complexities with background checks or the legal status of corporations, trusts or other legal entities.
Taken from the ATF website. The original post can be found here - https://www.atf.gov/firearms/qa/what-status-my-nfa-application-form