Flight Incheon International Airport departs from Seoul Incheon International Airport at 12:15 and arrives at Tokyo Haneda Airport at 14:40.
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 Saturday and it would be best to avoid Tuesday, as prices are on average 95.7% higher than on Saturday. (Average prices, based on 2452934 datapoints.)
We have a lot more statistics for flights OZ 104 and between Seoul and Tokyo.