how quickly will SK/US turn the tide NK could take the whole penninsula. It's a possibility. Whether they could keep it is unknown. But they will probably be stopped in a couple of days and the fighting will continue for probably three weeks until NK is essentially gone as a fighting force. At that point China should come in, not to drive the UN forces out as was attempted 50 years ago, but to administer the country of NK until unification with SK.
I'm not sure if they could take the whole of SK, it all depends of a lot of unknown factors really. I do think that it would take weeks rather than days to stall a NK advance. The SK's might hold them up in some areas, but there are weakpoints and NK can the expolit the terrain to find gaps.
Having said all that, I can't see this coming to war, not now anyway. It just doesn't feel right. What the NK's seem to be doing now is political, the key to their degree of success in a war lies in surprise. And right now, with talk of war all over the world media, is hardly a good time.
Look for an invasion when you least expect it.
On the other hand, if the US decides to launch an airstrike against NK's nuclear facilities...well...who knows.
Dangerous times.