Recent trip was in conjunction (unfortunately) with the Bangkok Shutdown Rally, we were advised to be at the airport 4hrs earlier - the most boring part was to spend almost half a day there without any activities or whatsoever. Nevertheless, thanks to Naraya and Duty Free shop plus the looooong lines to check-in, we did fulfill our 2.5hrs effectively ^^
The prayer room is located at the Departure Hall, far at the end. There are also seats specifically mentioned for Muslim Patrons only, just beside it. The room is spacious and larger than any of the airport's prayer room that I have been before. As usual, the sejadah (prayer mat) is available. For ladies, several telekung (clean and smells nice) are also at your convenient.
Note: Wudhu' (Ablution) area for ladies is facing the men prayer area covered with a thin bathroom curtain. If you are not comfortable with this setting, washroom is available across this room.