What are the requirements for the retirement visa extension deadlines in Thailand?

Just so I'm clear, if you have an O-A it just needs to be seasoned for 2 months prior to asking for your first extension and 3 months prior for extensions after that. For the O visa money must be in 3 months prior to your first extension.

So you are already ON an extension (which means this isn't your first yearly extension). That means the funds need to be IN your account 3 months BEFORE you apply for a new extension.

And depending on the immigration office you use you can apply for a new yearly extension between 45 days and 30 days BEFORE your current extension expires.

There is no reason to wait to the very end of your extension to apply for a new one because they ADD the year extension on to your expiration date.

BUT

you are correct IF your extension expires on the 17th of March you would need the money in the bank from the 17th of Dec to hit the 3 month seasoning on the date your current extension expires.