What is the time limit for bank statements for renewing a marriage visa in Thailand?

At most offices you can have the 2 month detailed statement and the bank letter up to 7 days before you show up for the extension.

You need to update your bank book the day you show up to apply for the extension

Seeing as there are over 80 offices in thailand you'll have to check with your specific office or say where you're getting the extension and see what people answer as far as how that office does the dating of the financial documents

Just so you're clear you need THREE things to show proof of funds when you go to get the extension

1 - standard bank letter (just a paper stating your name, p/p#, account #, date and the balance the day they write the letter)

2- two month detailed transaction report

3 - updated bank book and copies of the relevant pages the day you apply for the extension