If the nerve of a molar is "exposed", pain would indicate the nerve isn't dead yet? for how long would the nerve have to be exposed before it dies?

You need aRoot Canal: When the pulp of a tooth becomes damaged & the nerve is exposed, you need a root canal to save the tooth. That's when a dentist cleans out the pulp and nerve from inside the tooth and seals it back. Without that, the tooth will continue hurt & be sensitive to hot or cold. Even worse, if it is not treated, it can fill with bacteria from your mouth & form an abscess. Go see your dentist.

Tough to say: Truthfully, every situation is different. I've seen patients who had an exposed nerve for weeks to months and it was still alive.

Varys: If you are getting pain from and exposed nerve you will need to have a root canal to take care of it. Molars can give confusing symptoms because there are between 2 and 4 separate canals containing nerve tissue. One of the canals might be dead / dying while the rest are very irritated.