I have a friend who is attempting to fly out and back to Thailand the same day.  If you get to DMK approximately 08:30 with no bags, check in online, how long to clear Thai departures immigration?

 

Any particular day of the week that is busier than others?

 

I looked at other threads related to this, but most people were focused on arrival immigration clearance.

Difficult to say - in the past few months I have not had an outbound immigration queue any longer than 10 minutes

Sometimes it depends on who is going through at the time - if I see a lot of group tours (eg Chinese groups) queuing outside the immigration entry point for departures, then I go for a wander for 15 minutes and come back.