I imagine that, just as in 1950, they'd sustain as long as they were moving forward without any serious checks -- which would mean a few days, most likely. Just like any other army, morale isn't that big a problem if you're advancing.
Once the check comes, I think it's all down to a test of the effectiveness of NK's brainwashing, and that's a complete unknown. Worst-case would be that individual ideas of self-preservation have been suppressed.
The Japanese in WWII, and the Chinese in the Korean War, showed us that it's entirely possible to do that pretty effectively, and I think North Korea has probably gone far beyond what they were able to achieve.
My pessimistic view is that the crumbling will end up taking place for military reasons -- supply chains, destruction of assets and units, etc., and not because the NK soldiers just give up.
Interesting assessment!