Per Labor Code § 5903, a petition for reconsideration may be filed "[a]t any time within 20 days after the service of any final order, decision, or award made and filed by the appeals board or a workers' compensation judge." Althoug LC 5903 establishes a basic 20-day time period for filing a petition for reconsideration, most parties are given longer.
That's because the California Code of Regulations § 10605 extends the time to act following service of a document. It explains that if a document is served by mail, fax, email or any method other than personal service, the period of time for exercising any right or duty to act or respond will be extended: