Getting shot in the head can be deadly within seconds or can be deadly within a few hours or even days.
It all depends on the severity of the injury of the gunshot to the head.
If the gunshot is gonna cause death then you're likely gonna die within minutes to an hour of being shot in the head and if you can get medical attention soon enough then you might survive.
Getting shot in the head is a very severe injury and requires immediate medical attention and if no medical attention is summoned then you're likely to die soon after being shot in the head.
Sometimes when a person lives after being shot in the head they are then disabled due to a severe brain injury and some people have a good outcome and live a good normal life.
But then again some people die instantly or within a minute or two of being shot in the head.