Typically, you have up to 14 days to add a new vehicle to your policy. Every company is different so always check with your specific insurance company for their guidelines. However, if you do not add the car to your current insurance and happen to get into an accident before your new policy starts, your old insurance company will not cover the damages since the car was never added. If you're interested in learning more about car insurance for a new car, see here.
If you have any further questions, don't hesitate to ask