If you are a visitor over 18 with a valid driver's license from your home state, you may drive in CA for as long as you like without obtaining a California Driver's License. However, if you are between 16-18 years old then you may only drive with your out-of-state license for 10 days. After 10 days, you must have a CA driver's license or a Nonresident Minor's Certificate, which is issued by the DMV office) with proof of financial responsibility.
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How long can I drive in CA with my out-of-state license if I am a visitor?