How long must the 800,000 THB remain in a Thai bank account before applying for a visa and how long after obtaining it?

Scroll a few days back, and you will find answers.

Not sure if you are asking about getting the initial Non O visa in country or a year extension of stay.

If the former most offices don’t require seasoning of funds prior to applying for the Non O. Jomtien does require 2 months prior. Some offices will insist that you show the funds came from overseas. Some don’t seem to care.

For the extension it’s 2 months prior and 3 months after and the balance can’t go below 400k for the rest of the year. It would need to be back up to 800k 2 months prior to your next application.