Flight Don Mueang International Airport departs from Bangkok Don Mueang International Airport at 20:20 and arrives at Colombo Bandaranaike International Airport at 22:15.
The flighttime is about 3h 25m. The distance travelled ("as the bird flies"): 2300 Km. So the average speed is roughly 673 Km.
There is an 1.5 hour time difference between Bangkok and Colombo