The period of time in which you should receive your refund from an auto insurance company for removing a vehicle from your policy will generally be dictated by state statutes or laws. Many states require insurance companies to send you a refund for their unearned premiums within 30 days.
For example, Florida has state statute 627.7283 which says that the unearned premium must be returned within 30 days of the company's receipt of the written request for cancellation, or the company must pay interest at the rate of 8%. If a premium finance company is involved, the company is responsible to refund to the premium finance company within 30 days.
You should check with your state's insurance regulatory agency to find out what law your state has set forth regarding auto insurance refunds.