When I’m going abroad then I’m booking a bus ticket a bus ticket 5 hours before the flight.
The official bus time is 2 hours and 30 minuets. But once it took me 3 hours and 30 minutes. It was long queues at the check in and when I came to the gate then it was time for boarding.
When it’s “normal” then the complete transfer process at the airport takes 15-30 minuets but occasionally one hour.
So summary I recommend 5 hours.
Book the bus ticket at least 3 Days before, I recommend one week. Now I was buying it a week before and then the bus was filled up to 70%. It’s Chuseok so many Koreans are going abroad. Update: If you is taking a morning flight the first day then it will take some hours to pass the security. I was going at 5 pm and then there was no queues. The bus ticket can be booked online or at the bus station. I’m buying at the government complex bus stop. The cost is 23.100 won.
Just one example: I was speaking with a Canadian woman once and she had to wait 2 hours, just because all the busses were full. I have no clue if she made it or not but I would have been very nervous.