Well whatever China decides to do in unlikely case of an all out invasion of S.Korea(ROK) by North(DPRK), it has no more then three days, at most to do so. North Korea doesn't have capability to threaten US homeland directly. Not in a conventional sense, even with nuclear weapons. They can, or try, to threaten some of our Pacific bases, with ballistic missiles. However those are small point defense areas likely to have ballistic missile shield.
In case of conventional invasion, timing would like this;
Day1: North Korean Army (InmunGun) rolls towards Seoul, forking into three main thrusts. With respect to North, left thrust goes for Seoul, Middle races as far into South as possible,right goes for the coast. Special forces will attempt insertion at various points with various means. North Korea has many small submarines, simple landing craft, wooden airplanes, and some Western helicopters. If successfull both right and middle thrusts continue towards Busan. Allied airpower concentrates on counter-battery strikes , targeting long range artillery, missile assets, and supply interdiction. Allied ground forces and tactical aviation (helicopters, A-10's, ROK F-16's), concentrates on close air suppourt and anti-armour. Long range USAF bombers go after fuel and ammunition depots, and supply columns. North Korea has outdated air defense.
Day 2: Hopefully the North Korean steamroller would be running out of steam, going short on fuel, perhaps also on large caliber ammunition. However DPRK infantry and surving special forces may still be well supplied, using captured S.Korean or Allied stores. A big if. DPRK heavy armour will be definatly ground down by this point. Allied aviation assets continue to pound supply lines.
Day 3: DPRK mechanized forces are out of fuel and ammunition. DPRK forces South of DMZ consolidate and prepare for S.Korean/Allied counter offensive. Air power attrits them continuously.
Day 4: Prepositioned Allied mechanized reinforcements start arriving. At this point DPRK leadership has to retreat, call for cease fire, or go nuclear. The may have leverage of holding S. Korean, and Japanese civilian population as 'nuclear' hostages while they try to extracticate their remaining forces from S. Korea.
Day: 5 War ends with a cease fire in best case, or a regional nuclear conflagration at worst.