State laws differ, as do insurance companies' guidelines, regarding if there is a grace period between a due date and an insurance policy cancellation. If you do not pay your car insurance premium by the due date in some states you are canceled as of that day.
In other states there may be a grace period or a number of days in which the company must give you notice of cancellation. For example, in New Jersey if your insurance company cancels your policy for non-payment you should receive a 15 day warning notice.
In Florida, the insurance company needs to give 10 days notice of cancellation of an auto insurance policy. This is typically done by insurance companies mailing bills to policyholders at least 10 days before the due date advising that the policy will cancel on a specific date unless the premium is paid. These companies may then cancel the policy of those who fail to send payments on time.
You can check with your State's Insurance Regulator to determine the specific rules that apply in your state for how long after the due date your car insurance can be cancelled if the premium is not paid.