For the initial in-country Non-O visa based on Retirement that you are applying for at your local immigration office, the funds aren't required to be sit in your bank account for a minimum period.
It has to be in your bank account at least two months prior to your application day (not the day of your extension starts) for your 1-year extension.
You need to go to your local immigration office to get the handout of the requirements for the In-country Non-O visa application AND 1-year extension because those are office specific.
But here is the general information for in-country Non-O retirement requirements and attached is general info for a 1-year extension.
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/9.FOR-RETIREMENT-PURPOSES-50-YEARS-OLD-NON-O.pdf