Flight Incheon International Airport departs from Seoul Incheon International Airport at 07:25 and arrives at Tokyo Narita International Airport at 09:55.
The flighttime is about 2h 30m. The distance travelled ("as the bird flies"): 1555 Km. So the average speed is roughly 622 Km.
The best day to arrive at Seoul is Thursday and it would be best to avoid Saturday, as prices are on average 202.4% higher than on Thursday. (Average prices, based on 6763293 datapoints.)
We have a lot more statistics for flights LJ 201 and between Seoul and Tokyo.