How many months prior to my retirement visa expiration should I maintain 800K THB in my account for renewal?

You are on an extension, not a visa anymore.

You have to have 800K thb in your bank account 2 months prior to the application date, not the last day of your current extension. Ensure you had 800K in your bank account until around the end of February (for 3 months after you were granted the current extension), and you have had at least 400K in your account since then.

Here is the general information about an extension application based on retirement. You NEED to go to your local immigration office to get the latest handout for the requirements just before you apply for next year's extension.