California produced about 2 billion pounds of beef last year. That comes out to 8% of the US market for beef, the vast majority of which comes from the US.

The numbers I found said that each pound of beef requires anywhere between 1000 and 5000 gallons of water. (I suspect the high end is probably too high, so let's go with the low end.) That gives us 2 trillion gallons of water used in California each year for beef production.

Back of the napkin: If 10% of people who get their beef from California replace it chicken (which uses about a half the water per pound, both are about 1,000 calories per pound), the state would save 100 billion gallons of water per year. That's about 3 days worth of water for the whole state.

(Apologies for lack of sources, I didn't realize this would turn into a research piece, so I didn't save them, but they seemed reputable.)