If a person was in a car accident and hurt their brain/head and have a lot of pressure how long for them to keep living or it will go away soon when?

It depends on the: Damage happened, sometimes it go away fast the other it may take longer time.

It depends: The duration of headaches after a head injury of any nature depends somewhat on the severity of the injury, and whether there was trauma inside the skull (usually seen on a ct scan). If there is no trauma, then roughly 90% of individuals will be headache free 3 months after a concussion. When headaches go beyond this time frame, in many cases it is because of overconsumption of headache medication.