3 days, maybe you could get by with a gallon a day. It takes that long to dehydrate to death (on average) without any water.
Having lived the real deal, I keep 60 gallons minimum on hand in the house for my first 3 days. I can do it on 5 a day. It sucks. Hard.
/johnny
I'm not saying one gallon is optimal or even bearable. But it is survivable under the conditions currently in the region.