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.
TLDR : Answer Summary
If you plan on taking an ATK Covid test at Suvarnabhumi airport, expect to spend 1-2 hours at the testing area. The cost of an ATK test is 550 baht, and results are typically available within 20 minutes, though there's a waiting time of about 1 hour. For RT-PCR tests, they are available as well but at a higher price (around 5000 baht for results in 3 hours). The testing area is located outside near the taxi lineup. Be sure to allow adequate time for testing before your flight to avoid any last-minute issues.