What are the requirements for a Thai wife visa extension regarding the bank statement and processing time at Bangkok Bank?

Drop 20 baht into the account so that an up to date transaction shows on the bank book update

Usually when I go to my SCB branch at Central Festival and tell them I need the bank statement for my visa extension they already know what I mean and print exactly what I need. I don't know if it's the same with BKK bank though. (I peeked on the bank employee screen and they have like a folder with both the statement and letter of income for immigration purposes ready to print... )

As stated above, you need a 2-month statement (I do 3 months just to be sure) - but you also need a guarantor letter signed by the branch manager or someone of sufficient authority. These can be done up to 7 days before you apply for the extension. Also, you will need to update your bank book on *the same day* of your application, so before going to immigration deposit 100 baht and update the book to show current 400k+ balance.

As Tod says , they will just look back at the required 2 months and they circle the date on your copy. Bangkok Bank are usually very good at giving your the letter on the spot (. Maybe 20 minute wait ) . I got mine at the weekend and then took that in on the Monday .

bangkok bank usually tells people that they can print 6 months on the spot but if you need a year transaction history it takes a week (although I got a year printed on the spot at the B/B near my house :O)

You shouldn't need a full year of transaction history because there is no requirement that the 400K baht stay in your account AFTER you get the extension granted. The seasoning rules are only that the money needs to be in there for 2 months before you apply. I'd wager that the 6 months transaction history they can print on the spot for you will be more than sufficient as long as you have the bank letter and update your bank book on the date you go apply.