There is no set percentage for a contingency fee in representation of competent adults (there are rules regulating contingency fees for representation of minors and incompetent adults), except that it cannot be "unconscionable." I have seen the percentage range from 25% to 50%. Generally, the more complex the case and the more costs expected to be advanced by the attorney (in other words, the more risk an attorney is taking that he or she will not receive any fee), the higher the percentage. It's also not uncommon to see steps, for example, one percentage if the case settles or is otherwise resolved by a certain event in the litigation (such as before the first trial date set by the court) and a higher percentage if the case settles or is otherwise resolved after the specified event (which is justified because an attorney will expect to put in significantly more work after that event). It's our firm's practice (whether the case is in San Diego County or Sonoma County) to charge a lower contingency fee if the client is willing to pay the costs as the case progresses (including filing fees, expert witness fees, and deposition costs) because then the client is sharing in more of the risk.
I hope this was helpful, and good luck!