No idea. I doubt they would have designed it to fail after a short time. And, by definition, "emergency" means "something very bad needs help very quickly", so I don't see any reason why you would worry about how many hours or days later you'd want that number to be reachable. In the 0.0001% situation where it might be an issue, you could just hang up and call 911 again. In all the years that the service has been available, not one person has ever posted that this became an issue.