Cal. Civ. Proc. Code § 1297.132

Failing any agreement referred to in Section 1297.131, a party which intends to challenge an arbitrator shall, within 15 days after becoming aware of the constitution of the arbitral tribunal or after becoming aware of any circumstances referred to in Sections 1297.124 and 1297.125, whichever shall be later, send a written statement of the reasons for the challenge to the arbitral tribunal.