Flight Suvarnabhumi Airport departs from Bangkok Suvarnabhumi Airport at 22:55 and arrives at Busan Gimhae International Airport at 06:30.
The flighttime is about 5h 35m. The distance travelled ("as the bird flies"): 3243 Km. So the average speed is roughly 581 Km.
There is an 2 hour time difference between Bangkok and Busan