We do know, though. There's the science behind it. Sodium Nitrite impedes the delivery of oxygen in your body. From the wiki:
"Sodium Nitrite, NaNO2, acts as a catalyst in the conversion of the hemoglobin in your blood into methemoglobin (MetHB), a molecule with a much higher affinity with oxygen. This occurs when the ferrous ions in the regular hemoglobin are converted into ferric ones. Since its affinity is so high, methemoglobin cannot let the oxygen flow into other tissues that need it, thus depriving them of oxygen even while you're breathing. Death, then, occurs by hypoxia."
So this means that you could be found well after your brain starts resembling a turnip. If you're found before that, you could manage to luck out of it after a hospital stay. There's a cure for Sodium Nitrite poisoning, but there's no cure for organ damage.
And from everything I've read, no one consumes the recommended dosage and decides to back out by taking themselves to the hospital. They're usually found passing in and out of consciousness in a heap on the floor. A few have managed to dial emergency services themselves, but even that'll be a challenge with hypoxia.
