California Code, Revenue and Taxation Code - RTC § 19517

(a) In the case of income received or accrued during the lifetime of a decedent, or by his or her estate during the period of administration, or by a trust, the Franchise Tax Board shall mail notices proposing to assess the tax, and shall commence any proceeding in court without assessment for the collection of the tax, within 18 months after written request therefor (filed after the return is made) by the fiduciary of the estate or trust or by any other person liable for the tax or any portion thereof.

(b) After filing a request pursuant to subdivision (a), a fiduciary may consent in writing to waive the limitation prescribed by subdivision (a).