If you’re on ADOS and close to 20 AFS, when do you drop your retirement packet?

MAJ (Join to see), assuming this is a continuation of your previous question*, you never answered if you have applied for, and been granted, Sanctuary status or not. Your question indicates that you have not as retirement processing is all part of that program*.

If you plan on having a regular retirement (i.e., "Active Duty" retirement), then apply for Sanctuary.

If you don't and want to process a non-regular retirement (I have no idea why you wouldn't go that route considering your situation), then contact your applicable Retirement Service Officer* and they will guide you the rest of the way.
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* https://www.rallypoint.com/answers/can-you-retire-with-20-years-afs-plus-20-years-reserve-time-before-age-60
* https://www.hrc.army.mil/content/Officer%20Sanctuary%20Program
* Retirement Service Officer - https://soldierforlife.army.mil/Retirement/rso

Army policies may differ, but in the Air Force Reserves we can drop our retirement packet no earlier than 1 year out from when you want to retire and no later than 6 months prior. I have been told by a few retirement experts and HR personnel that they do not recommend picking the date that is exactly at 20 years in case there is a miscalculation, and you are short. They have seen that happen before and it is extremely difficult to fix. Additionally, they may not process everything until you actually hit the 20-year mark, and it is confirmed that you have the required time served.