Ok, so first question:
-It works by changing the electic charge of Fe ions in hemoglobin, wich is Fe2+, to Fe3+. This way the new hemoglobin (wich is called methemoglobin) can't bind oxygen atoms to itself and can't carry it to other cells
As far as quickness...
-If everything is done correctly you should be dead in no more than 45 minutes and lose consciousness in something like 15.
Common suggestion for this method are starting using anti-emetics a couple days prior to the attempt (they should be dopamine blockers), not eathing anything 8 hours prior to taking the SN, using medication to reduce stomach acidity and a strong dose of anti emetics 1hr/30minutes before ingesting the salt.
15-20g of SN in 100ml of water should be an optimal dose.