Most applications are decided within 72 hours. If the result is a denial, the practical position is that you cannot enter under the visa-waiver route and must apply for a visa instead.
Reapplying rarely helps
Submitting the same details again will normally produce the same outcome. If something was entered incorrectly — a passport number, a date of birth, a name spelled differently from the passport — that is worth correcting. Otherwise, the route is a visa application at a Korean diplomatic mission.
Do not travel to test it
Airlines check the status before boarding. Arriving at the airport with a denial on file means being refused at check-in, not at the border.
A denial is a decision about this application, not a permanent bar. A visa application is assessed separately and on its own documents.
Source: K-ETA official portal
More about K-ETA
What is K-ETA
K-ETA is an electronic travel authorisation for visa-free travellers to South Korea. What it is, who it applies to, and why many nationalities do not need it right now.
Do I need K-ETA?
Many nationalities are temporarily exempt from K-ETA. How to check your own passport, and what applies if you are not exempt.
How to apply
Applying for K-ETA on the official site: what you need in front of you, where people get stuck, and what happens after you submit.
Cost
What K-ETA actually costs on the official site, why third-party sites charge more, and when the cost is zero.
What you need
What you must have ready to complete a K-ETA application, and the photo and name rules that cause most rejections.
Processing time
Official guidance on K-ETA processing, what makes it slower, and why no paid service can speed it up.