What is the rule regarding how much one must deposit each month to obtain a Retirement Visa Extension?  Is it 65,000 baht for each month?  Specifically, is it an average of all deposits for the year or a different rule?

Afaik there is no country wide rule yet, some immigration offices might accept monthly deposits, other's don't. So you would have to ask your local immigration office for the details

Thank you Jack.

There is no rule about showing 65k going into a Thai bank instead of using proof of income from a embassy yet.