What are the regulations regarding the 2/3 month seasoning for the 800,000 baht retirement extension in Thailand?

You need to have funds of 800k baht in your Thai bank account for full 2 calendar months as of application day for a new one-year extension. Some offices ask for 3 months instead of 2 months.

“3 months after the extension granted” is counted from the day you get your new extension stamp at many offices. But there are a few offices that count “3 months after” from the day your new extension starts.

You need to check the province specific detailed policy with your local immigration policy.

It's to show PROOF of your having the 800,000 Thai baht on deposit in your Thai bank for the 2 full months before the current ending date of your one year extension of stay,

...and kept at the 800,000 Thai baht balance for 3 months after you receive the one year extension of stay STAMP in your passport.

Otherwise, the bank account balance must be at least 400,000 Thai baht for all other months.