Flight Incheon International Airport departs from Seoul Incheon International Airport at 08:05 and arrives at Tokyo Narita International Airport at 10:30.
The flighttime is about 2h 25m. The distance travelled ("as the bird flies"): 1555 Km. So the average speed is roughly 643 Km.
The best day to arrive at Seoul is Monday and it would be best to avoid Wednesday, as prices are on average 141.9% higher than on Monday. (Average prices, based on 6774780 datapoints.)
We have a lot more statistics for flights LJ 217 and between Seoul and Tokyo.
