How long does the 400,000 baht need to be held in a bank for a guardian of a child visa in Thailand?

and just like that we're done. Good luck to the O/P and make sure that you ask the immigration office that you're using what their policy is on it :)

If you are talking about getting a year extension based on raising a half thai child using the 400K baht banked money method there is no "official" seasoning required on the funds <- meaning that there is nothing written IN the immigration rules that state the money needs to be seasoned.

All the rule says is that the 400K baht has to be in a thai bank account in your name only on the date you apply