How long does an NCO who got busted from E6 to E5 on an Article 15 need to wait to gain back their rank?

I do recall two different times we had NCO's take "Voluntary Reductions" versus UCMJ and I think what it does is restore your TIG and/or DOR when you are again able to get promoted. Now I say this as it was done with AGR NCO's and the promotion system is different requiring promotion recommendations from commanders to even be considered but it let them re-attain their previously held ranks faster if they performed worthy of the recommendation. Unsure if the AD system would allow a voluntary reduction to be considered earlier then meeting TIG requirements. Can't recall all the details as I have been retired for a couple years now and when these happened they were a few years before that.

Did you go to the board? Or you had it back for your TIS?

I'm currently going through this myself. I am an E6 with 25 months TIG and am receiving a Field Grade AR15 and as punishment getting 45 days Extra Duty, Forfeiture of pay for 2 months and reduction in grade from E6-E5. I have appealed it to my next higher COC stating that the reduction in rank will effect me financially however it is out of my hands at the moment and up to the COL for some leniency. It is my understanding from what I can find in AR 600–8–19 that my time will start over once orders are published for my reduction and that my reduction will happen immediately on the date the LTC BN commander gave me my reading. Then it is like I just got promoted to E5 and will have to follow the guidance of promotion criteria to be eligible for E6 promotion "Page 34" of AR 600-8-19, Table 3-1.

Depending on conduct and performance I could be boarded in secondary zone since I will already have the required TIS easily, and 6month TIG as an E5. I already would have completed DLC2 and I actually have already competed DLC3 as a SSG. I would assume that after a board appearance and a "GO" and have points that I would be promoted back to E6 the following month on the promotion list, orders would be published, however WOULD my DOR for E6 be the new DOR ... OR would it be reverted back to my ORIGINAL DOR when I made E6 in JAN 2020... and would have to start TIG for E6 all over again?

Also, since on your ERB it would show DOR for E5 promotion I'm also assuming that is not what is based off of. Example being if I am Demoted to E5 but was promoted to E5 in 2019, I would not automatically have say 36mo TIG? Since I'm essentially being demoted and starting that rank fresh. I'm almost certain this is not the case.

It is not the end of the world as I only have 6.5 years TIS and although I am a bit down I know I can bounce back and learn from this. It seems that LTC Burch have bounced back and it has not hindered your career.