Flight Narita International Airport departs from Tokyo Narita International Airport at 11:45 and arrives at Seoul Incheon International Airport at 14:10.
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 Tokyo is Wednesday and it would be best to avoid Tuesday, as prices are on average 238% higher than on Wednesday. (Average prices, based on 4524252 datapoints.)
We have a lot more statistics for flights LJ 204 and between Tokyo and Seoul.
