There are many analysis of potential conflict with North Korea. In general the expert opinion is that North Korea would be mostly defeated within 4 to 8 weeks.
* China does not intervene on the side of North Korea
* North Korea will likely try to fully utilize chemical and biological weapons
* North Korea will likely not be able to use its few nuclear weapons
* North Korea has extensive tunnels and about 100,000 committed troops for a long mess after the main conflict
With conventional artillery, tanks and forces, North Korea could likely inflict 100,000 to 200,000 casualties on South Korea.
With chemical weapons the South Korea casualties could double.
South Korea and the USA have bunker busters and would be able to rapidly establish air superiority. South Korea has artillery and better tanks.
north Korea also has 70 submarines with missiles that would have to be hunted down. The submarines have missiles and torpedoes to cause trouble for commercial ships and other targets.