The applicable Labor Code Section is Section 510. However, the answer to your question depends on which IWC wage order applies. Generally, unless you're in the agricultural industry (the largest exception), hours worked in excess of 8 hours in one day are paid at time and a half, as are hours worked in excess of 40 in a week. Assuming that IWC Wage Order 4 applies to you (professional, technical, clerical, mechanical and similar occupations), you would be entitled to 4 hours of overtime for each of the 5 days of each work week, for a total of 20 hours of overtime. The purpose of the 40 hour per week period for overtime is simply to allow for the payment of overtime when an employee works 8 hours or less per day but has more than 40 hours in a work week (e.g., you work 8 hours for five days and then 3 hours on a Saturday). You can find all of the IWC wage orders on line. Just google Industrial Welfare Commission Wage Orders.
The answer here will also depend on whether you bring a claim under the federal FLSA or the California state Labor Code. In California, any time worked over 8 hours a day is overtime, and any time worked more than 40 hours in a week is overtime. Thus, for the first 3 days and four hours, you would get overtime for the time worked more than 8 hours in the day. At the point you reach 40 hours in the workweek, all time thereafter is at overtime.
Good luck to you.
This answer should not be construed to create any attorney-client relationship. Such a relationship can be formed only through the mutual execution of an attorney-client agreement. The answer given is based on the extremely limited facts provided and the proper course of action might change significantly with the introduction of other facts. All who read this answer should not rely on the answer to govern their conduct. Please seek the advice of competent counsel after disclosing all facts to that attorney. This answer is intended for California residents only. The answering party is only licensed to practice in the State of California.