When do I need to deposit the 400,000 THB for my Non Imm O visa extension application in Thailand?

The funds need to be in the account for two months before the date you APPLY for the extension (not from when your current one ends) SO when it needs to be in to meet the seasoning requirements depends on WHEN you're going to go apply for the next yearly extension. At most offices you can apply when you have 30 days or less left on your current stamp SO if you do that you need the funds in the account THREE full months before you go apply (as you'd apply a month before the extension expires)

Suggest your make your deposit more than 2 FULL MONTHS before you make your application.

The 400,000 Thai baht MUST be on Deposit in a Thai bank account in your name only TWO FULL MONTHS BEFORE you submit your application for the one year extension of stay as Married to Thai.