There's a percentage of people who fail SN. For the most part, people fail for two reasons: 1. They get found by a third party, taken to hospital, given the antidote and they survive; 2. They vomit extremely heavily, resulting in minimal SN being in their system, so they don't CTB.
No one knows what percentage of people fail as we are talking about suicide here and therefore any scientific studies would be unethical: you can't conduct a randomised control trial with SN!