Flight Incheon International Airport departs from Seoul Incheon International Airport at 13:00 and arrives at Tokyo Narita International Airport at 15:25.
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 Thursday and it would be best to avoid Tuesday, as prices are on average 103.97% higher than on Thursday. (Average prices, based on 6931119 datapoints.)
We have a lot more statistics for flights KE 711 and between Seoul and Tokyo.