not an expert (sic) but i think the antidote (methylene blue) works pretty fast and complete once administered. however it relies on the cardiovascular system distributing it throughout the body, especially to the brain. so if the heart has already failed, emergency personell need to distribute it via cpr, which of course reduces the chance of suvival. my guess is that the point where survival becomes extremely unlikely is reached after 8-12 minutes after not having been administered the antidote (NOTE: THIS IS AN UNEDUCATED GUESS).
the mechanism is as follows: the SN oxidizes the iron-atom in the red blood cells. This turns the functional hemoglobin into dysfunctional so called "Methemoglobin" that is no longer able to bind oxygen and supply it to the body (the antidote simply reduces it back to functional hemoglobin). now suffocation begins. consciousness will be lost within a minute or two. One will probably experience fear and severe discomfort. when the organs run out of oxygen, the body starts dying. brain damage sets on after a couple minutes. irreversible brain damage is reached after some 20-30 minutes.
there is probably a small chance that if one's found quickly, one will be reanimated with possibly severe brain damage.