Flight Don Mueang International Airport departs from Bangkok Don Mueang International Airport at 18:00 and arrives at Changsha Huanghua International Airport at 22:20.
The flighttime is about 3h 20m. The distance travelled ("as the bird flies"): 1466 Km. So the average speed is roughly 440 Km.
There is an 1 hour time difference between Bangkok and Changsha