For Thai retirement visa extension you should file for an extension one month before your visa expires. You must always check with your local immigration office for further information on when you may request for an extension. As long as you've been granted permission to remain, you may seek for an extension. Even if your application is complete, you should submit it a few days before the expiration date in case you need to supply extra documentation.
As with the Non-Immigrant O visa, you must complete a TM7 Extension of Stay form to achieve this. This time, you must establish that you have at least 800,000 Thai Baht in your Thai bank account (or in your country of residency if you apply in that country), 65,000 Thai Baht each month in your bank account, or a combination of the two that totals at least 800,000 Thai Baht.
Retirement Visa Extension Document Requirements