2 Answers

The rule of threes:

3 minutes without oxygen
3 days without water
3 weeks without food
and you die, on average.

On the other hand, if water is the only liquid (with no liquid in food- fruits veggies, juicy meats, etc) the average adult won't last more than 3 days.  Children and most elderly will die sooner.  This is why people who fast are allowed to drink water.  And the average adult can last a few weeks or longer eating nothing and drinking water (this depends on how much someone weighs prior to the fast and the composition of that weight fat/ muscle/bone/fluid).  However, no health professional would recommend a prolonged fast...more than a day or two...and none of us would tell anyone to withhold fluids for an entire day (we r talking healthy adults).  AND babies/children, pregnant women, and most elderly should both eat and drink fluids daily.