In the United States, the length of time a dealer is required to keep insurance on a vehicle after it is purchased varies from state to state. Typically, dealers are required to maintain insurance on vehicles until the buyer either takes possession of the vehicle or the dealer's insurance policy expires, whichever comes first. Dealers typically provide temporary tags or dealer plates for the vehicle, allowing you to drive it legally while you wait for your permanent registration and plates.
Please note that these requirements may vary depending on the specific laws in your state. It's always best to check with your local Department of Motor Vehicles (DMV) for accurate information regarding dealer insurance requirements in your area.