1 US gal of water roughly 3.78 litres is the minimum I would say, not only for drinking but food prep, brushing teeth, light face washing, etc.

I don't know what the status in Washington is but across the border in Vancouver they have a boil water advisory on that will likely continue for several more days. At one time it extended to greater Vancouver (2 million) but has since been reduced to about 1 million people involved. That's easily the largest "boil water advisory" ever given in Canada. They've reccommended a min. of 4 litres per person per day which means the stocks of bottled water are being drained very quickly and there is a lot of stinky people walking around Vancouver in dirty clothes!

BTW, the advisory was initiated because recent heavy rains have washed a lot of material into the water which makes the water appear very cloudy and muddy. Because the water plants can't guarantee the safety of the water, large bacterial organisms may still be present with the sediment so everybody boils water. To compound the situation, certain areas have been without power which reduces the ability of a lot of people to even boil it for usage.