Flight Incheon International Airport departs from Seoul Incheon International Airport at 13:35 and arrives at Tokyo Narita International Airport at 15:55.
The flighttime is about 2h 20m. The distance travelled ("as the bird flies"): 1555 Km. So the average speed is roughly 666 Km.
The best day to arrive at Seoul is Friday and it would be best to avoid Thursday, as prices are on average 161.26% higher than on Friday. (Average prices, based on 6738419 datapoints.)
We have a lot more statistics for flights JL 5250 and between Seoul and Tokyo.