Flight Incheon International Airport departs from Seoul Incheon International Airport at 07:50 and arrives at Tokyo Narita International Airport at 10:30.
The flighttime is about 2h 40m. The distance travelled ("as the bird flies"): 1555 Km. So the average speed is roughly 583 Km.
The best day to arrive at Seoul is Sunday and it would be best to avoid Friday, as prices are on average 174.42% higher than on Sunday. (Average prices, based on 5620171 datapoints.)
We have a lot more statistics for flights RS 705 and between Seoul and Tokyo.