How long before my marriage visa in Thailand expires should I go to extend it with the required 400,000 THB in the bank?

To extend your marriage visa in Thailand, ensure that you have 400,000 THB in your bank account for at least 2 months prior to your application. Ideally, visit the immigration office 2-4 weeks before your visa expires to allow sufficient processing time, although you can apply on the exact expiration date as the latest option. It is also necessary to keep the funds in your account until your visa extension is finalized.