It is good to have enough on hand to be able to lie low initially.
Don't forget, your hot water tank usually has 40-60 gallons of water in it. Just flush the sediment out 1-2 times per year using a garden hose and the lower access valve. I's though about putting two tanks in series, one active (for hot water) and the other inactive, as an extra storage tank. That way the water would be continually refreshed with clean water every time you turned on the hot water.
== H2O in ==> Spare tank ==> Hot water tank => faucets
Also consider getting a good water filter like the Gravidyn Filter by Katadyn.
I have the large Gravidyn and a portable "hiking" filter.
Add to that several packages of pool shock (to make bleach) I could treat thousands of gallons of water if I had to.