This is what the rule book says (page 70):
LETHAL INJURIES: Some critical injuries are lethal, meaning they will kill you after a certain amount of time unless you are stabilized before it runs out. The time limit is decided by rolling a die, indicating the number of minutes, hours, or days you have left, as indicated for each injury. The GM makes this roll secretly. If someone attempts to stabilize you and succeeds at a roll for Medicine before the time limit runs out, you survive. The time limit also states what kind of medical equipment is necessary to help you, either basic (B) or advanced (A). Without the right equipment, there is nothing to be done.
Basic medical equipment (B) involves things a field medic would carry with them in a bag: scalpels, bandages, basic medication, disinfectants, and other similar objects. If you do not have basic
medical equipment, you cannot create it by tearing some sheets into bandages or sharpening your bread knife. You need real medical equipment. Advanced medical equipment (A) is more specialized
stuff that you find in a hospital. It could be a supply of blood, medicines, surgical equipment, or an operating table. Advanced medical equipment is hard to find, and you might need
some means of transportation, like a car, to move it around or bring people to it.
This means that when you take a lethal injury (Lethal: Yes) you will die sooner or later if nothing is done. How long it takes until you die is not revealed to you. The GM simply rolls a die and looks at the table to see if the number indicates minutes, hours or days before you die. For example, if you take Spinal Injury and the GM rolls 3, it means that you will die in 3 hours. The GM does not tell you this but someone making a successful Medicine roll will be able to see how long you have before you die.
The way to avoid dying is to get stabilized by a successful skill roll for Medicine before the time runs out. But to be able to even get to roll for Medicine your caretaker needs to have the right equipment, basic medical equipment or advanced medical equipment. Basic medical equipment is quite easy to find. Advanced medical equipment is almost impossible to find. This means that many times a critical injury could be a death sentence - how do you find advanced medical equipment in three hours, in this world? Other times finding advanced medical equipment in, for example 4 days, is the start of a "scenario" or a mission, where your life is on the line.