Probably the same rule as it is for Brits, and dependent on the way that a particular Immigration Office interprets them. For instance, our local Immigration Office insists on either 800,000 baht being in a Thai Bank account for 3 months prior to extension date, or 65,000 baht a month for 12 months prior to extension date.i.e. :-
(1) The alien has obtained a temporary visa (NON-IM); and (2) The applicant is 50 years of age or over; and (3) Proof of income of not less than Baht 65,000 per month; or (4) Account deposit with a bank in Thailand of not less than Baht 800,000 as shown in bank account transactions for the past 3 months; or (5) Annual income plus bank account deposit totaling not less than Baht 800,000 as of the filing date of application
However, our local Immigration Office will not accept (5), only (3) or (4), and also small "peculiarities" that I believe are not required by other Immigration Offices e.g. photograph of you standing outside your house with the house number clearly visible?