What is the timeline for applying for a 1-year extension on a Non-O visa based on retirement in Thailand?

you are confusing two different things.

GETTING a Non-O visa issued inside the country from Chiang Mai requires a minimum of 21 days left on your stamp because of the under review period to get the Non-O visa

BUT

When you go apply for a yearly extension (or any extension up in Chiang Mai) you can do it when you have 45 days or LESS left on your current stamp

So if you have a stamp good until Nov 8th the soonest you can apply for the year extension was any time AFTER Sept 24th.

As long as you meet the seasoning requirements for the proof of funds you can go any time between now and when your stamp runs out.

You don't lose any time going early as they add the year extension to your current expiration

AND

there is no under consideration, retirement extensions are issued on the spot