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.
Yes that's about right. It depends on where you are. On a very hot day in a desert you can die in less than a day. I can't find any exact numbers for this though.