Flight Incheon International Airport departs from Seoul Incheon International Airport at 21:30 and arrives at Tokyo Narita International Airport at 23:25.
The flighttime is about 1h 55m. The distance travelled ("as the bird flies"): 1555 Km. So the average speed is roughly 811 Km.
The best day to arrive at Seoul is Friday and it would be best to avoid Sunday, as prices are on average 131.29% higher than on Friday. (Average prices, based on 6739192 datapoints.)
We have a lot more statistics for flights OZ 178 and between Seoul and Tokyo.