When a tooth actually dies is an unpredictable event because it could take days, weeks, months, or even years before it becomes a dead tooth. In other words, sometimes it is instantaneous while other times it is dying slowly.
You can try to pressure your dentist as much as you want but even if they give you an answer they would only be guessing. There is simply no way of knowing when it will die. The only thing that we can do for you is to check if it is still alive at every 6 month check up.
Why it takes time to die
There are many conditions which may try to kill your tooth but it won't die immediately. Your body has its own immune system and the ability to repair itself from damages, both of which extend the life of your dentition.
Immune system. Your tooth will not succumb to an infection immediately. Your body will try to fight it off for as long as it can and sometimes it is successful in doing so. Other times it requires help from a dentist.
Self-repair. Teeth are able to repair itself and heal naturally to a certain extent. You can see this in how it is impossible to reverse small cavities via remineralization.
Essentially it is these two factors which account for the time discrepancy before it becomes a dead tooth. Sometimes it is unsuccessful but other times it is successful in fending for itself and does manage to escape from the grasps of death.
Causes of tooth necrosis
Conditions which may result in a dead tooth:
Tooth decay. Untreated cavities will progress through the enamel, dentin, and finally reach the pulp where it will kill the nerve. How does it feel to know that you could've prevented it from dying if you went for regular dental check ups? You could've stopped it at the enamel or even dentin stage.
Infection. A severe dental abscess can involve the tooth and end up killing your tooth.
Trauma. Any type of blunt force trauma to the mouth can kill the tooth instantaneously or cause it to die slowly over time. This happens quite frequently in sporting accidents an injuries, which is why it is important to wear a mouth guard.
Large restorations. Sometimes large restorations like fillings or crowns can cause the tooth to become non-vital after a long period of time. The pulp likes a certain amount of distance or buffer away from foreign objects. Being too close to restorations can cause it to be irritated and eventually perish.
Symptoms of a dying tooth
A tooth that is in the process of dying may not always be symptomatic because it can be asymptomatic. However if it were to act up, here are some signs and symptoms.
Spontaneous throbbing toothache. When the tooth nerve is about to die, you may experience spontaneous throbbing pain. That is a tell-tale sign of an unhealthy nerve that is approaching the doors of death.
Lingering toothache when eating/drinking. When subjected to a cold stimulus, a healthy tooth will feel sensitive for a brief moment. An unhealthy tooth will have a lingering sensation that lasts up to a minute or more when stimulated.
Discolored tooth. If you notice one of your teeth is darker than the adjacent teeth it could mean that it is dying or is already dead. The discoloration occurs because the blood supply gets cut off to the tooth so it no longer receives nutrients. It is similar to how dead people start turning grey and devoid of color.
How to tell when it is dead
Only your dentist can give you a definitive diagnosis of a non-vital tooth. There are a few methods which they use to verify it.
Vitality test. A cold, heat, or electric pulp test can be used to test if the nerve is still alive. All of these methods involve subjecting the tooth to a particular stimuli to see if it would respond. Dead teeth often have zero response to all of these tests.
X-ray. Necrotic teeth will often have a constricted canal, obliterated canal, or a halo around the root tip. All of these are signs of a tooth that have died or are in the process of doing so.
How to kill a tooth faster
There is no way for you to speed up the death of your own tooth. However if the tooth pain is unbearable you could have your dentist do it for you. There are two ways which will instantly kill your tooth.
Root canal. Instead of waiting for the nerve to die, your dentist can physically remove it from your tooth via a root canal procedure. Once the nerve is gone it will be considered dead.
Extraction. Alternatively, surgically removing the entire tooth from your mouth will also kill it. This is more invasive and extreme because once the extraction is a irreversible and you'll be left with a missing tooth afterwards.
Aside from these two techniques, there is no home remedy which can achieve the same effect. You either see your dentist or suffer through the pain and wait for it to die on its own.
It may take a tooth days, weeks, months, or even years before it finally dies. The process is unpredictable because your body can resist death by fighting back with its immune system and also attempting to repair itself. Both of these will delay the potential inevitability of pulp necrosis. Once in a while it does succeed and the tooth manages to survive!