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They'd dehydrate to death in a couple of days. The coma necessitates being fed and given water
There are a lot of variables. If they are just unconscious, they will die from dehydration alone in less than a week. If they overdosed they could throw up and choke on their own vomet, dying in a few minutes.
There are an infinite number of ways it could play out, but the longest you could live is as long as it takes a lack of water to kill you.
As others have said, they’d die without food or water.
But let’s assume they somehow were getting food and water but no medical treatment. At that point it would heavily depend on the nature of the coma. It’s not like movies where the person is there but unable to wake up, there are many different kinds of comas.
Some are a result of brain damage. In this case, depending on the damage, the person would probably die quickly from bleeding, infection, or swelling.
Drug overdose, assuming they don’t die from the overdose, could go many directions depending on the drug and the extent of the overdose.
Basically, there’s no one answer to your question. The coma that you see in movies where the person is fine but unable to wake up is actually quite rare.
In medical terms, a coma isn't one thing, but a scale from anything below normal brain function to death. But for what most people think of as a coma, it is when you are at the level of unconsciousness where you can't protect your airway. That means they can still breathe, but their ability to breathe can be blocked off easily, and they aren't able to correct it. So, a person in that type of coma could die very quickly. Things like being in the wrong position, swelling, fluids blocking the airway etc. In the case of overdose, their brain is not giving them the signal to breathe (opiate overdose), and they will definitely die quickly without intervention.
edit: there's a reason why the the first two ABCs of trauma are Airway and Breathing. Those are the things that will kill you if they're not working. Dehydration is way down on priority list. It's not going to be the thing that kills you in an untreated coma.
it's all about dehydration and choking risks.
without help, dehydration kills in about 3-4 days max.
It really depends what the cause of the coma was. I think you need to be more specific on the situations. Like diabetic coma, hemorrhagic stroke, sepsis, etc. but I think for most of the times, the person would come through or wake up (depending on age and pre existing medical conditions) to seek medical attention. There most likely will be life long lasting effects.
At most, they're living 3-4 days before dying of dehydration.
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