It seems that you are asking if you have turned in a vehicle and replaced it with a new one, how long do you have to inform your insurance company of this change. The answer will differ depending upon state laws. If there is not a specific state law regulating this then it would be up to the individual insurance company, their guidelines and the terms of your policy.
For example, the California Department of Insurance states that most insurance policies within CA provide 30 days automatic coverage for a vehicle that replaces a vehicle already on your policy.The coverage normally is the same coverage you had on your previous vehicle. Notify your broker-agent as soon as possible of any replacement vehicle. If you wish additional coverage, there is usually a requirement that you notify your agent or your company within a designated time period.
Some policies provide automatic coverage for a newly acquired vehicle that is an addition to the vehicles you already have on your policy. There are usually specific conditions that must be met. For example, the purchased vehicle must be reported to your agent or company within a designated time period (e.g., 30 days) or there may be a requirement that in order for coverage to automatically apply, all of your other owned vehicles must be insured with the company.
Do not assume that the coverage will be on your new vehicle for 30 days. For other states or insurance carriers there may be no grace period or it may be a shorter period such as 7 or 14 days.
If you plan on buying a vehicle that will need to be added to your current insurance policy, it is usually best to inform the insurance company before you go to pick up the car. Even if the automobile is not placed on the policy until later the insurance company should be able to advise you of the time limit you have to add it on the policy before there is no coverage for it.