Once every 18 months, and California measures it the strict way: from the date of the violation, not the date you finished the course or the date the court signed off.
The mechanism matters here. California traffic school doesn't dismiss your ticket; under Vehicle Code 1808.7, it masks the conviction, keeping it confidential so insurers and employers pulling your record don't see it. That confidentiality is what's rationed. Pick up a second eligible violation within 18 months of the first and the court can still let you attend traffic school, some do, but the second conviction shows on your record anyway, points and all. You'd be paying tuition for the certificate, not the benefit.
So the practical questions when a new ticket arrives: what was the violation date on your last masked ticket, and is this new violation date at least 18 months after it? Your court notice will usually tell you whether you're eligible, but courts process on their own information, and if you believe you're outside the window and the notice says otherwise, that's a call to the clerk before you pay anything.
Two permanent exclusions run at any frequency: commercial license holders can't mask, and violations serious enough to be ineligible, alcohol offenses and the like, stay ineligible.
Inside the window with a fresh ticket, the course won't protect your record, so weigh the fine-plus-points reality instead. Outside it, the trade is excellent, roughly $52 to $55 in court fees plus a course that can cost under $10, details on our cost page.
Every DMV-licensed online school we track is compared on the California page.