If North Korea fires an ICBM at the USA, how many minutes will it take to reach U.S. mainland? Within how many seconds will the U.S. detect it?

Any ballistic missile fired from the DPRK would be detected almost immediately by either US radar systems in South Korea, naval vessels in the Sea of Japan or satellites in orbit.

Once a weapon is detected, those systems will begin evaluating the trajectory of the missile which will help in determining the impact point. Depending on where that missile is headed, travel time would be in the neighborhood of 15–30 minutes. 
(ans by  Christopher Witman Ex-US Army and Reserve Veterans)