With the exception of serving military, if you are residing in this State for twenty days or longer (used to be 10), California Law says you must register your car here. That includes Students, people with other State residences, people here on short-term jobs, even people just visiting relatives!
My Brother-in-Law worked for the State Board of Equalization under Governor Deukmejian. Any time he saw a car with an out-of-state plate but another signifier that they lived here - USC sticker, UCLA, Dodgers, etc. - he recorded the plate number and reported it. Many people were visited by Law Enforcement personnel to ticket them for failure. It resulted in a HUGE fine in those days, upwards of $800.00.
A man in my church was ticketed for still having Oregon plates after starting work here two months earlier. His defense was that he still owned property in Oregon and his car was registered there. So, he was asked, "Where do you work?" When he told them he was given his ticket.
It is the Law here.
