Standard delivery takes 2-4 weeks. With extra post because of Christmas the whole international mail system starts going slower, so you'd expect the October earnings (sent the end of November) cheque to be delayed.
If it hasn't arrived by the 25th (Christmas Day), get in touch with Google and they will reissue the cheque.
Alternatively (depending on the amount involved) pay extra for secured express delivery (takes 5-7 days).
Unfortunately Korea is not on the list of countries that Google do direct bank transfers (sometimes called electronic funds transfer) which is below. If you do have a bank account and would prefer to be paid that way - let Google know - after a few years of nagging by various publishers they did provide it instead of cheques. Just out of interest is it North or South Korea you are referring to? :) :-
List of countries EFT is allowed to for Google Adsense publishers
Australia Austria Belgium Canada Czech Republic Denmark Finland France Germany Hungary Ireland Israel Italy Japan Netherlands New Zealand Norway Poland Portugal Slovakia Spain Sweden Switzerland Turkey United States United Kingdom