Cal. Rev. & Tax. Code § 38432

If the amount specified in the determination is not paid within 10 days after service of notice thereof upon the person against whom the determination is made, the amount becomes final at the expiration of the 10 days, and the delinquency penalty and the interest provided in Section 38451 shall attach to the amount of the tax or the amount of the tax required to be collected, unless a petition for redetermination is filed within the 10 days.