Short Answer: 90 days.
this is pasted email reply I got from ATF:
What does the surrender notification letter need to state?
Forms using the exemption § 479.105 (f) Discontinuance of business. To use the exemption the forms must have a letter for intent of discontinue of business. This letter must have the following to be considered a valid “OOB Letter.” Notice that in doing this your SOT will be flagged as going out of business.
· The letter has to be dated the day you decide to go out of business.
· The target date of when the business will be closed has to be included within the body of the letter.
· The letter cannot be submitted more than 90 days before going out of business.
· Must be on letter head.
· Reference the SOT(s) going out of business, list all FFL(s) associated with that SOT.
· List all machineguns in the inventory that will be requested transfer under provision 479.105(f).
Can this letter simply be attached to an eForm 3 under the upload documents screen or does it need to be sent in differently? Yes
What is the timeline between surrender date and transfer of a Post Sample? example: if they simply let their SOT expire at renewal can they transfer the Post Sample now, 6 months early? (Another SOT told me the No Law Letter window is 90 days) Correct
To fill in the rest of the blanks, the SOT attached his OOB letter as "other" attachment on the last page of eForm3. Submitted Jan18th. Approved today. He submitted 2 the MG form3's Sturday. Also approved today.
I called the SOT specialist back at ATF for clarification. She told me that when an SOT that is a sole proprietor surrenders SOT that the ATF will still approve their pending Form4's. The idea is that the NFA item in question is still in the former SOT's possession and even though they aren't still an SOT they can transfer to the new owner. its a little convoluted and kinda seems to violate the letter of the law, but its nice that a form4 that has been baking for 8 months doesn't get shitcanned but it's SOT goes out of the game.
My local ATF investigator noticed that he missed a call from me and called me back in the meantime. he told me that sometimes they have trouble navigating weird situations like that but that they are open to helping facilitate the doing of business without anybody getting their dicks stepped on. All in all, both the Forum and the ATF was pretty responsive and easy to deal with all around.
PS: I may be interested in your wares. hope this helps. you can message me if you need clarfiifcaton. my deal went through smooth.