If U.S. troops were to pull out of South Korea...

[Yorpheus]

...how long do you estimate it would take for North Korea to begin an invasion?

Post-invasion with the North and the South now unified but under Communist rule, how long would it take for the country as a whole to return to a Democracy?

[jester5150]

30 minutes upon confirmation of final troops withdrawal to begin invasion if that.

To return to democracy, it would take as long as it takes for a pro demo country to invade and take control of unified Korea (if possible) and bring about a "regime change"