Cal. Rev. & Tax. Code § 6537

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, unless a petition for redetermination is filed within the 10 days, and the delinquency penalty and the interest provided in Section 6591 shall attach to the amount of the tax or the amount of the tax required to be collected.