Flight Incheon International Airport departs from Seoul Incheon International Airport at 18:35 and arrives at Tokyo Haneda Airport at 21:00.
The flighttime is about 2h 25m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 615 Km.
The best day to arrive at Seoul is Tuesday and it would be best to avoid Saturday, as prices are on average 9.27% higher than on Tuesday. (Average prices, based on 2442293 datapoints.)
We have a lot more statistics for flights GA 108 and between Seoul and Tokyo.