@Ruth ******
My experience is with the Chaengwattana Immigration office in Bangkok to get a 1 year extension. I would obtain bank documents before going. They are valid for 7 days. I updated my bank book downstairs using the machine that is available even when the bank a not open. I would arrive around 8am. The office opens at 8:30am. I usually got my queue number 15 minutes later. I typically got my extension completed within 2 hours. I didn’t use and agent or make an appointment.
If you are also getting a re-entry permit then you are going to have to get in line again.
Avoid going the day after immigration has shut down for holidays. Note there are a lot of holidays in April.
Just get there early when the office opens at 8:30am.