Pioneering, brilliant tour company
I am writing as the author of the Bradt Guide to North Korea, and Koryo were instrumental in getting it done with accurate, up-to-date information, but they are the true pioneers of tours into the country, are very professional, their guides have outstanding relations with the local guides, and they really know what works - and what doesn't. You'd need a solid week at the very least, 10 days or more preferable to see Pyongyang, Kaesong, and the further flung regions (compare the port of Chongjin with the trading town of Pyongsong, for example). It's intense, you're in a group, bussed everywhere, there's an itinerary to follow - but, listen to the guides, let them do their job, and they'll open up and you'll learn so much, and you'll leave as friends, and if you ditch the cynicism and see what's actually being shown, you learn loads. It's quite a story.