GP listed a range. 2 days probably won't kill most people if they're starting at a healthy point and aren't too active or in an arid environment, but it will probably kill a lot of people if they start off somewhat dehydrated or are too active (by necessity or stupidity) during that period. The usual number tossed out is 3 days, but it's not like people will magically all drop dead after 72 hours from their last drop of water. It's an average or expected value.