In general Forms W-8BEN, W-8ECI and W-8EXP are valid beginning on the date the form is signed by the client and expires on December 31 of the third succeeding year.
For example, a Form W-8BEN signed on March 31, 2008 would be valid from the date of signature until December 31, 2011.
These forms also become invalid in case of a change in circumstances that makes the information on the form incorrect.
Form W-8IMY generally remains valid indefinitely, and becomes invalid only when the status of the person providing the form changes in a way relevant to the Form W-8IMY or circumstances change that make the information on the Form W-8IMY no longer correct.
Form W-8BEN, W-8ECI and W-8EXP that has a valid US tax number (ITIN, SSN, EIN) is valid indefinitely.