How much? 1 gallon/person/day for 2 weeks = 112 water bottles/person
In what? Clean and sanitized food-grade containers or PETE plastic (the big blue barrels or soda bottles --not milk jugs)
How Long? Until expiration for commercially packed
Or refill every 6 months for other containers
Purification? Clarify then Boil, Bleach or Filter --tablets and filters are available at preparedness stores and camping stores.
I know that blue barrels go on sale at Macey's each year during their caselot sale. I believe that falls in March and October.
This water would be used in emergencies not just for drinking but also for some sanitation purposes (depending on how desperate things are)