I was always taught that it was polite to send a thank you letter / email within 24 hours of having the interview. I would have thought that interviewers would try to let you know as soon as they made their decision.
Whether they decide to hire you or not, it's common courtesy for them to let you know. If they have decided not to hire you, it's nice to be able to know this so that you can cross them off your list. If they are going to hire you then it's also great to know so that you can stop looking for a job.
A common problem here in Korea is that places take a while to get back to you even when they do want to hire you. They might wait a couple weeks or even months. Then many people have already accepted other jobs and the school has to go through the process again.
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