If you are planning on taking an certificated ATK Covid test at Suvarnabhumi airport, I would suggest to expect 1-2 hours at the testing area.
We travel tomorrow , but our hotel is not far from airport so went today for the pre departure test.
ATK tests are 550bt each and we had our negative letter 20 minutes after the swab was taken, BUT had to queue for around 1 hour, so was there 1.5 hours in total.
They just require your passport and the payment.