Flight Incheon International Airport departs from Seoul Incheon International Airport at 09:20 and arrives at Tokyo Narita International Airport at 11:50.
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 Seoul is Wednesday and it would be best to avoid Monday, as prices are on average 106.55% higher than on Wednesday. (Average prices, based on 6928314 datapoints.)
We have a lot more statistics for flights RS 701 and between Seoul and Tokyo.