There is no certain time frame in which reversible pulpitis becomes irreversible pulpitis. Dr. Richard Mitchell states that it is more or less a matter of when you first notice the pain and how quickly you can see a dentist.
If the nerve in the tooth has a chance of recovering, dentists classify the inflammation as "reversible pulpitis," and something can be done to get the tooth back to normal, notes Dr. Mitchell. However, if the pressure from the inflammation has damaged the nerve, then it is "irreversible pulpitis." The nerve inevitably ends up dying, which requires a root canal or a tooth extraction.