The minimum amount of time needed to transit at Incheon Airport is as follows. However, it is recommended that you allow yourself more time than listed below for hassle-free boarding.
Domestic route to domestic route: 40 min.
Domestic route to international route: 1 Hour 40 Minutes
International route to domestic route: 1 Hour 40 Minutes
International route to international route: 1 Hour 10 Minutes
When receiving your boarding pass at the airport of your city of departure, in theory, you do not need to exchange your boarding pass for a new boarding pass in order to board your connecting flight. However, in cases in which the information on your transit boarding pass significantly differs from the information of the related airline, you may be asked at the boarding gate to exchange your existing boarding pass for a new one to board your connecting flight.
Passengers from visa-exempt countries and Korean visa holders are free to leave the airport after completing the immigration procedure. However, under certain circumstances, an immigration officer may ask a visitor to complete an interview or show proof of sufficient funds for entry into the Republic of Korea.
Source: Flyasiana.com / December 2016