How long does it take to receive a 1-year extension for a non-immigrant retirement visa (O) at the Phuket immigration office?

I did mine last Wednesday at the Phuket office. I made an appointment online, so they were expecting me. They seem to know me there after 20 years. It took me about 15 minutes to submit the documents and pay. He said come back and pick up my passport the next day between 4 and 5 PM. I live in Rawai and didn't want to drive there and back at that time of day so I went in after lunch on Friday, made a copy and I had the rest of the paperwork to get my re-entry permit over at the new building. That took about a half an hour.

Retirement extensions are done in a single day as long as you meet the requirements.