California law is that front side windows must allow 70+% of light in. Back windows and rear window can be any darkness.
THere is this
http://www.tintcenter.com site has the official CA laws as well as laws for all the states.
California Car Tint Laws
Most cops in CA won't ever pull you over for front window violations. I have 35% in the front window, and cops don't give me a second glance; however, one thing to remember is that if you do get good tint with lifetime warranty.
Many window film manufacturers void this warranty if the darkness is not legal in your state. Just something to keep in mind.
-- harrison