Thanks for your input. The first link you sent is not the CA Franchise Tax Board - it's a site for vehicle registraton refunds, etc., it seems...? "The California Department of Tax and Fee Administration (CDTFA) administers California's sales and use, fuel, tobacco, alcohol, and cannabis taxes, as well as a variety of other taxes and fees that fund specific state programs."
I did find the info finally on the FTB site for income tax amendments and refunds, and it's 4 years after the original due date:
https://www.ftb.ca.gov/help/disagree-or-resolve-an-issue/claim-for-refund.html
