Hi,
Here at Prachuap I.O, the rules are 12 x 65,000thb. On arrival at the office for renewal of extension, you have to submit a recently updated bank book, a 1-year bank statement, (showing 12 monthly incoming credits of 65,000thb or more), and a payment advice note showing that the payments have been internationally received and also the relevant forms and photos. With Bangkok Bank, you have to give 7 days' notice for these documents, but I believe other Thai banks will do them quicker. I am also sure other I.O's and offices have different rules, but this seems the norm here for over 3 years.