Flight Incheon International Airport departs from Seoul Incheon International Airport at 21:10 and arrives at Tokyo Haneda Airport at 23:30.
The flighttime is about 2h 20m. The distance travelled ("as the bird flies"): 1487 Km. So the average speed is roughly 637 Km.
The best day to arrive at Seoul is Sunday and it would be best to avoid Monday, as prices are on average 610.62% higher than on Sunday. (Average prices, based on 2460111 datapoints.)
We have a lot more statistics for flights NH 178 and between Seoul and Tokyo.