In that the actual blast from a nuclear bomb will annihilate an area up to several miles, a person living within the immediate area of the blast zone will not have to be concerned about heading promptly to an underground bomb shelter, obviously.
It’s people like you and me (hopefully) that will survive the initial blast. Our greatest concern is radioactive fallout. Fallout will kill as many, if not much more than the blast itself. And how long you have before fallout arrives depends on three things:
Distance – The more distance between you and the source of the radiation, the better. This could be evacuation or remaining indoors to minimize exposure.
Shielding – The more heavy, dense material between you and the source of the radiation, the better.
Time – Most radioactivity loses its strength fairly quickly.
A typical concern of most people is protection from the explosion. Of course, knowing where the detonation is going to occur may be revealed when the government realizes the country is being attacked via missile. Average ICBM travel time from North Korea to the west coast of the United States, for example is about 30 minutes.
Even though the missile is tracked as soon as it leaves the launch pad, it’s likely that NORAD will be so busy tracking the missile and taking actions to blow it out of the atmosphere that common folk like you and I won’t receive sufficient warning, if any. This is especially true if the recently cancelled civil defense nuclear warning and shelter maintenance plan once common to every town, school, and local government isn’t reinstated and understood by the public.
On the other hand, a terrorist-delivered nuclear weapon (or chemical/biological weapon) will obviously provide no warning. Your best defense is distance. If you are several miles or more away from ground zero, at best you can react to the bright flash of light. If you are immediately keen on what’s going on, you may have several minutes to reach the safety of your shelter before the blast wave arrives.
This scenario would apply primarily not to people who are at ground zero (they will be long gone, of course), but to those who are between about 3 and 10 or more miles away. And they would have mere seconds to react – about 3 to 15 seconds respectively.