What is the waiting time for Korean investors for I-526 approval?

See USCIS case processing times after the end of March may go faster as they are prioritizing cases with a current priority date which includes Korea.

The usual processing time for Korean nationals is about 24 months. If you filed in March 2018, you should not be far behind for consideration.

If the case is well-prepared and the project has no issues, you should expect to hear in the next few months.

An I-526 for Korean would be prioritized for processing as visas are available. As to the processing time, no one knows.

I-526 petitions are currently estimated to take two to four years. Some filed with more established projects seem to take slightly less, and there are even a few projects approved for expedited processing where petitions can take as little as a few months. I-526 petitioning, starting soon, will actually consider the nationality of the investor, as those subject to visa backlogs will be held longer in favor of petitions filed from nationals of countries without visa backlogs.