How long does it take to extend a retirement visa at Phangna Immigration and obtain a bank letter?

From your (edited) post, you're using Phangna Immigration, which is one of the islands (I believe). You'll be better off asking this question in one of the many islander groups, they would know the specific in and outs of that immigration office.

You might want to state what immigration office you are using. Is this your first one year extension?

There is the bank letter, which should be good for 7 days. The letter is different from a bank statement which you might also need. Also bring your bank book update the same day you apply.

As long as you have all the required documents correctly, you will get a 1-year extension stamp onto your passport on the spot (while you are waiting for the receipt) and leave the immigration office with it. How long you need to wait there depends on the immigration office and how busy they are..

IMPORTANT REMINDER: In this group, we don't talk or mention visa agents, sorry 🙁

We usually get the bank letters the day before we go to immigration. The next day we present our request package and get the extension immediately.

>Just wondering how long does it take to extend your retirement visa after submitting it?

In Thailand, GENERALLY the immigration office will grant an "under consideration" stamp the same day you submit your extension of stay request. The "under consideration" is like a temporary extension of stay. When you return to the immigration office, you will either be granted the one year extension of stay for the balance of the time, or (highly unlikely) an extension of stay denied stamp, and you will have to leave Thailand in 7 days.

HOWEVER, some extension of stays do not have an "under consideration" period. I never done an extension of stay based on retirement, so this may not apply to you. If the one year extension of stay is granted on the stamp, then you can stay for another year.

>Also how long before can you get the letter from the bank?

Depends on the immigration office. MOST allow it up to 5 days old.