If the fronts are low you need to replace them, especially with the rains were are experiencing in California. The tire shop should put the tires with the most tread depth on the rear so they will rotate them for you. The reason is that traction needs to be maintained at the rear since it is much easier to control front traction loss compared to the rears (you can steer with the front wheels but not so much with the rears when you're fishtailing).
Anyway, to answer your question it depends. I have an old RWD Model S and the front and rear tires wore evenly. I went 60K miles on my last set of tires without rotating them. I'll try rotating this current set every 10K miles (same Goodyear Eagle RS-A2 model as the old ones) and see if I can eke out any further distance.