I start with 1 gallon per person per day, but store roughly twice what I expect to need. My stockpile now is almost all cases holding 3 one gallon bottles from WalMart. The bottles are those HDPE "milk jug" material which is fairly fragile, but the bottles just sit on a shelf, inside the cardboard boxes, so they're pretty protected. I don't rotate that water at all and some of it is approaching 2 years old now. I'll probably rotate it at the 3 or 4 year mark more because I'm worried about the plastic bottles holding up. I think a 3 gallon case is about $1.80 right now, so it's hard to beat the price. I used to store water in rinsed out 2 liter soda bottles and would consider it again just because of how much sturdier they are. They're just hard to stack and move unless you borrow some supermarket bottle trays.

I have 18 boxes or 54 gallons for my family of four. I also typically fill up some 7 and 5 gallon containers (and the bathtubs) when a hurricane approaches. Even in the worst likely case, that gives me enough for my family with plenty left for another family for a week.