Coverage for an additional vehicle varies from state to state and insurance company to insurance company. Your state may or may not offer a grace period for coverage. So, I cannot guarantee you are insured to drive your newly acquired vehicle. The only way to know for certain is to contact your state's Department of Financial Services (or Department of Insurance). Also, you will want to contact your insurance company. They will not be able to offer a grace period that is less than your state requires. But, they may offer a grace period that exceeds what your state requires.
Remember, you can click here to inquire about California auto insurance law and get an insurance quote for any state in the United States to compare your rates.