"Six minutes to Respond" to a North Korean missile launch
This concept is a dangerous Cold War relic that has very little relevance to U.S. vs North Korea.
When we were facing off against the Soviet Union, there was a fear of thousands of incoming thermonuclear warheads in a first strike designed to annihilate our ability to retaliate. At that time, against that size of nuclear force, the concern was that the President would need to make a decision to launch retaliation before the Soviet missiles arrived. Launch on warning is a very dangerous strategy because it significantly raises the risk of accidental war from a false alert.
This situation does not apply to North Korea. Even with worst-case estimates, their entire nuclear arsenal could not threaten our ability to retaliate - it would have to be hundreds of times larger and far more sophisticated. Even for a true superpower, it is really doubtful that a disarming first strike was ever a realistic scenario.
If we detected a launch towards the U.S., South Korea or Japan, there would be little to do except wait to see where it landed and what kind of warhead - if any it carried and then respond appropriately. A massive nuclear strike against North Korea would not stop a launch already in flight.