Flight Narita International Airport departs from Tokyo Narita International Airport at 18:25 and arrives at Seoul Incheon International Airport at 20:55.
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 Tokyo is Wednesday and it would be best to avoid Sunday, as prices are on average 281.81% higher than on Wednesday. (Average prices, based on 4486639 datapoints.)
We have a lot more statistics for flights LJ 210 and between Tokyo and Seoul.
