How long do I need to keep 800k THB in the bank when using a non-O retirement visa in Thailand before switching to monthly income?

for the 800k banked money method you need the funds in the account for 2 months before you APPLY for the year extension, the funds need to be in the account for 3 months after your non-o stamp expires <-(the date the extension actually starts not from the date you applied for it) AND you need to keep 400K baht in the bank the rest of the year until you go to apply for the next extension showing 65K baht a month in transfers for the previous 12 months.

It's tough to go from banked money to monthly income because of the seasoning requirements for the whole year you're on the extension AND the need to bring in the money every month as well for the next extension.