The next bus is at 8:00. It's not always every 25 min; they run more buses during peak times and fewer at off-peak. You can find the schedule at http://www.kallimousine.com/eng/schedule_en.asp.

You will also want to pay attention to which terminal you are coming in. If you are landing in the main terminal (typically when using KE or OZ metal) you will save about 15 min than if you land in the satellite terminal (most other carriers including KE/OZ code-shares). Since this is in the OZ forum, I will assume you are flying on OZ metal.

I fly into ICN 20 times a year or so, and I would say a typical wait for Korean immigration on a non-Korean passport is 10-15 min or so. Maybe 3 times a year I'll get unlucky and it will be 30-45 min.

Luggage comes quickly at ICN and OZ does a good job of prioritizing. If you're flying on OZ metal in J, your checked bags will be out typically around 20-25 minutes after you get off the plane. (again if you're coming into the satellite terminal, it may take a while longer)

Customs is VERY quick at ICN. Shouldn't take longer than 30 seconds. 1 time in 100, I've had my bag x-rayed and even then the entire process was 2 min.

If your flight is not delayed, I would agree with Rhubarb that you have a good chance of making the 7:35 bus on the 6:50 flight. (6:50 from the satellite terminal would be pushing it, but possible.) If you want to be safe, take the 6:30.