I went to Bangkok Immigration today to purchase a Multiple-entry permit (TM8). Got there when the office opened after lunch and was out of there at 2:30pm. It actually would have taken less than 90 minutes but I got my number (C264), went to the copy center, ate lunch, and did some shopping. When I got back I missed my number. Had to get another (C297). Still "only" 90 minutes.
TLDR : Answer Summary
The individual successfully obtained a Multiple-entry permit (TM8) at Bangkok Immigration, taking approximately 90 minutes for the entire process. This included a lunch break and some shopping, though they missed their initial number and had to get a new one.