How long is the validity period for the bank's annual statement for a Non-O visa extension for retirees in Bangkok?

Not quite

@Gre***

, IF you update your bank book every month during the previous year it will show the monthly balance and you won't need a year print out of transaction history. BUT what happens is IF you wait and update your bank book say every other month or so AND there are a lot of transactions on it, all it prints is a summary of money in/money out NOT a line by line history and immigration cannot tell from the bank book alone IF you followed the seasoning requirements. SO that's why they just ask that people get the year transaction history to check that you kept the 800K baht in the account for 3 months after the extension, and then never let the balance go below 400K baht until you topped it back off at 800K baht two months before your current application

That is the year's worth of transaction history, and most banks don't print it on the spot (some take a week to get it for you) and as long as the other bank letter is within 7 days AND you upate your bank book on the date you apply you won't have any problem with that years worth of print out being done before the application date.

Make sure you go to your bank and ask them how long it takes to get the full year print out. Many banks only keep 6 months of it on file at the bank and have to request it from the main office (which takes 3 to 5 days).