A person can live much longer without food than they can without water. Having an adequate water supply on hand for emergency purposes is as essential as having a food supply. Most bottled water comes with a two year “expiration date” This gives a timeframe pre-stamped on the bottle letting you know when to drink it and replace your supply.
So, how much water do you need to store? Generally plan on a 1:1:1 rule. The average person will need One gallon, per person, per day. Plan what you think is appropriate based upon those guidelines.