Flight Gimpo International Airport departs from Seoul Gimpo International Airport at 20:30 and arrives at Jeju City International Airport at 21:40.
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 Tuesday and it would be best to avoid Sunday, as prices are on average 797.15% higher than on Tuesday. (Average prices, based on 438140 datapoints.)
We have a lot more statistics for flights KE 1227 and between Seoul and Jeju City.