How long must the 800,000 Baht be maintained in a Thai bank account for a one-year visa extension?

If you are talking about the seasoning of funds for getting a yearly EXTENSION inside the country

SOME immigration offices give you a paper with a date on it 3 months from the date you get your year extension telling you when you update your bank books and come back to show them the balance on the account didn't drop below 800K

Now with that being said; only a few immigration offices do that. SO if you got your yearly extension BUT weren't required to go back after the 3 months and show your bank book still have the funds they will look at your bank balances the following year to see that you met the 3 months after seasoning requirement.

As long as you can prove that the combined total of your accounts (even if it wasn't the one you used to get the extension originally) had 800K baht in it for 3 months after the extension was granted you will be okay.

But lets start a the begin. It seems to me you do not have a visa and you do not have an Extension of Stay and are just trying to figure out how things may work. Where are you at the moment, if abroad which country or when inside Thailand which visa do you use?

if you have to visit the Immigration again to show the bank balance and it is not there, it might just be a cancellation of the Extension of Stay If you are not asked to make a visit the chance of getting denied a new Extension of Stay after this one comes to the end is very small.

No need to keep it in the bank you got your visa. If you did apply for an Extension of Stay at your local Immigration Office it is at least 2 months prior the application date until at least 3 months after the application date and after that not less than 400.000 Thb for the rest of the year. And it must me in a Thai commercial bank account on your name

There may be slight regional variations, but in general it's 2 months before to 3 months after ... must be in 1 account and NEVER fall below 400,000.