How far in advance should I contact a Korean university's international office about arrival and dormitory check-in?

Email the international office four to six weeks before you fly, not a few days before. What you actually need from them is three specific things: the exact date and hour dormitory check-in opens, whether early check-in is possible and at what cost, and whether they run any airport pickup for your intake. Pickup is not standard. Some universities arrange a bus from Incheon when a large group arrives on the same day, but many do nothing at all, and the only way to know is to ask directly.

If you land before check-in opens, the usual options near a Gwangju campus are a guesthouse, a motel, or a 고시원 for a very short stay. Motels are cheaper than they sound in Korea and perfectly normal for a night or two, and guesthouses near university areas are used to international students arriving early. Book before you fly rather than hoping to find something on arrival with luggage.

Getting there matters too. From Incheon airport the limousine bus straight to Gwangju takes about four hours and is the simplest option with heavy bags. Going by train means the airport railway to Seoul and then KTX from Yongsan, which is faster but involves changing with your luggage. Try to land on a weekday if you can, because offices and dormitory staff are not available at weekends and arriving Saturday morning to a locked building is a miserable start.

While you have the international office on email, ask two more things. First, whether the school requires a tuberculosis test or health check before dormitory entry, since a number of universities do and it is easier to know in advance. Second, when they run the group alien registration application on campus. Registration has to be completed within ninety days of arrival, and the campus group application is far easier than going to the immigration office alone.