I made an appointment for 8 AM, showed up at 7:50 AM at the American Services service window in the annex (no signs but look for a walkway on the right-hand side of the open-air lobby where all the Thai people are queued for visas). I had filled out my DS-4085 at home, handed that with my passport and appointment print-out in at the window. Was told it would be one hour, ended up being 15 minutes. I'd recommend 8:00 AM as this is just after the 7:30 opening rush but not late enough for things to back up. Allow 10 minutes to clear security.