According to the Financial Industry Regulatory Authority (formerly the National Association of Securities Dealers), you have two years from the termination date indicated on your most recent U-5 form.
In order to prevent your license's expiration, you must regain employment at another member firm within two years. If after two years you have not found employment with a FINRA member firm, you will be required to rewrite your Series 7 examination.
The firm with which you were previously employed must submit form U-5 to FINRA. So, if you are unsure of your specific termination date, contact FINRA