Flight Narita International Airport departs from Tokyo Narita International Airport at 13:20 and arrives at Seoul Incheon International Airport at 15:50.
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 Tokyo is Sunday and it would be best to avoid Monday, as prices are on average 1233% higher than on Sunday. (Average prices, based on 3830253 datapoints.)
We have a lot more statistics for flights NH 6971 and between Tokyo and Seoul.