Depending on what time arrive at ICN it can up to 90 minutes to get to downtown Seoul from the airport.(not including waiting for the bus).Busses usually run every 15-20 minutes or so. Taxis can take about an hour or less but cost $50 vs about $10 for the bus one way. That being said, here is one site to check out.

http://www.oriengate.net/k-sighting.htm