The rooms are available 24 hours. When I arrived they were all occupied. A few hours later I got one for 4 hours (min. stay), cost $40, then in 2 hour segments, same price structure. You might look at the BKK airport site and get the phone number for the Louis Tavern rooms and make a reservation.

PS Be sure you don't go through passport control when you land. Go to the transit area, the passport area will be on your left. You should continue walking straight along the hallway when you see the passport folks on your left.