Store 2 weeks supply of drinking water per person.
I store my drinking water in washed and bleached PET bottles, filled with tap water. Our tap water here is fantastic, so there is no need for me to purify it.
Keep your stored water our of sunlight and away from concrete floors if you can. The bottles can soak up the smells of things around them and possible toxins. Sunlight can release a toxin in the plastic of the bottle thus getting into your water.
Check on your water and change it every 6 months if needs be. I store both drinking water and washing water. The drinking water I change frequently and the washing water I don't have to worry about.
I also store some containers (like the one pictured) of store bought water due to them being easy to pour and easy to take camping - just grab and go.