This is not a legal practice. This practice is in direct violation of California Labor Laws. Employers are required by California Law to keep accurate payroll records that reflect all hours an employee has worked.
Employees may maintain relief if the violation by the employer is knowingly and intentional. You may also be able to recover costs in the prosecution of any such suit, as well as attorney fees. Under California law, employers must pay one and a half times an employee's “regular rate” if he or she works more than 40 hours per week or more than 8 hours per day; and double the “regular rate” for work in excess of 12 hours per day. See Cal. Lab.Code § 510(a).
The issue in determining the hours you worked is whether the employee was permitted by the employer to work.The law says that "it is immaterial whether it was necessary for (the employee) to work long hours, so long as he did, with the actual or constructive knowledge of his employer.” See Donovan v. Kentwood Develop. Co., Inc. (1982) 549 F. Supp. 480, 488.
Based on what you describe, it's is likely that you have a legitimate claim for failure to pay overtime compensation. You should contact an attorney in your area to discuss this. It would be very helpful if you kept record or at least could approximate how many hours you actually worked and weren't paid for.