I don't think that there's any rule about that.

If any individual feels that suffering, physical pain, or disability due to an injury or disease is sufficiently lowering his/her quality of life, then s/he is fully justified in auto-euthanasia. ...and is rightfully entitled to physician-assisted auto-euthanasia.

Michael Ossipoff

I do not know, and I hope none of us finds out.

But suppose you had suffered such pain a year ago, and survived against your will, would you regret your survival from the position of not being now in pain?

Really really bad, and permanent (with no reprieve or reward of any kind).

"How bad and long lasting does pain have to be for death to be good?"

Pain can be subjectively rated; to some extent it can be described. It's not possible to rate pain objectively, nor describe it objectively. The best one can do is create a subjective consensus about the kinds of pain and what subjectively severe pain does to one life.

I've experienced what I would call moderate pain quite a few times, some have a fairly long (2 months) duration. I could stand that kind of pain. I've experienced severe pain (burns, post surgical pain, broken bones) quite a few times, but the pain did not last long. If this severe pain was expected to last a long time, I wouldn't be able to hold out indefinitely.

There are two kinds of pain that I find difficult to tolerate: Very sharp, very strong 'stinging' pain (like that from a burn) or very strong dull-type pain accompanied by the feeling of 'coming apart'. Lower back pain might be an example -- not just pain, but the feeling of one's back being dislocated (which, of course, it wasn't).

It's hard to say how much pain is necessary for survival to be lexically removed. I can't give you a precise amount but I would say that any amount of pain that would make you wish you hadn't woken up that day would be enough to make death a good.

This would seem to make the amount of pain a lot less than you might have had in mind.