Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 08:55 and arrives at Jeju City International Airport at 10:05.
The flighttime is about 1h 10m. The distance travelled ("as the bird flies"): 271 Km. So the average speed is roughly 232 Km.
The best day to arrive at Seoul is Saturday and it would be best to avoid Sunday, as prices are on average 142.76% higher than on Saturday. (Average prices, based on 607084 datapoints.)
We have a lot more statistics for flights KE 1043 and between Seoul and Jeju City.