Best info I have on that subject is from the CIA World Book of Facts, with census data from 1995:
Christian 26.3% (Protestant 19.7%, Roman Catholic 6.6%), Buddhist 23.2%, other or unknown 1.3%, none 49.3% (1995 census)
How few years, I guess, is the question. They would have to have about doubled in the past 12 years... Possible, I suppose.
Yes, that information for the CIA handbook is out of date. However, I was overoptimistic myself. Churches like those of Dr. Paul Yong-gi Cho were growing so fast in the 1990s, I thought they would have crossed the line by now.
I just checked the 2006 edition of Microsoft Encarta. According to it, South Korea is 41% Christian, 16% indigenous beliefs (which I assume means ancestor worship and homegrown sects like Tongkhak), 15% Buddhist, 11% Confucian, 2% nonreligious and 15% other. Accurate figures are hard to come by because the Koreans take the same spiritual attitude as the Chinese; they consider it okay to belong to more than one religion at the same time.