Depends on your opinion of when you think they actually began putting emphasis on missile technology, rather than nuclear capability alone. Some will say 20 years, I've been of the opinion that it's been more along the lines of the last 10-7 years, if some projections of their spending are any indication
I'm of the opposite opinion, I feel they have been working seriously on their missile technology a lot longer than their nuclear weapons. It appeared to me that they had swung a deal with the Pakistanis, Khan et al would do the nuclear work and the DPRK would do the delivery system.
From the perspective of the North, missiles were a lot easier to sell to other countries as they could do so longer without the world coming down on them. After the fall of the Soviets, I think their biggest concern was finding ways to acquire money, and missiles provided such a way.
While I don't think their scientists are top notch, they are probably okay. To me a bigger issue is their ability to manufacture items to the level of accuracy required for such complicated items. I don't think they have it.
This post was edited on 1/24 at 9:44 pm