Storing Water in 1gallon milk jugs is a bad idea. Usually over 1-2 years the plastic will deteriorate, and the jugs will burst open flooding you house with water.
With that being said: You water bill is a good gauge of your current water consumption. Most people use about 100 gallons per day, or 300 gallons each day per household. Yes you can survive on 1-2 gallons a day during an emergency, but most people have never shut there water off for a week and tried it. It's tough.
Get yourself a good way to filter water and locate the nearest abundant water source. I know these are not the answer you are looking for, but in essences you are asking how long is a piece of string. If I had to determine a practical number that is obtainable for most families, I would say at least 250 gallons per household (rotated regularly). Something like this
is a good start, and can be placed in the garage next to your water heater.
Here are some additional figures I found;
Bath A full tub is about 36 gallons.
Shower 2 gallons per minute. Old shower heads use as much as 5 gallons per minute.
Teeth brushing <1 gallon, especially if water is turned off while brushing. Newer bath faucets use about 1 gallon per minute, whereas older models use over 2 gallons.
Hands/face washing 1 gallon
Face/leg shaving 1 gallon
Dishwasher 4 to 10 gallons/load, depending of efficiency of dishwasher
Dishwashing by hand: 20 gallons. Newer kitchen faucets use about 2.2 gallons per minutes, whereas older faucets use more.
Clothes washer 25 gallons/load for newer washers. Older models use about 40 gallons per load.
Toilet flush 3 gallons. Most all new toilets use 1.6 gallons per flush, but many older toilets used about 4 gallons.
Glasses of water drunk 8 oz. per glass (did you remember to drink your 8 glasses of water today?)
Outdoor watering 5 to 10 gallons per minute
"I guess you broke into the wrong goddamn rec room, didn't you" -Burt- Tremors
WINTERGEDDON: IX, X, XI, XIII, XIV, XV, XVI, XVII
SUMMERGEDDON: IX, X, XII, XIII, XIV, XV, XVI
SUMMER BUG OUT: XIII