The following is from the Westjet website:
Travel Bank credits from changed or cancelled flights can be used to pay for flights up to 12 months from the date your flight was cancelled. Sign in to your WestJet account to check your credit expiry date. Travel can occur after the expiry date. Regular Travel Bank credit can be extended for 12 months one time only for a fee. Please phone our contact centre to request an extension. This does not apply to an already extended credit. Expired credits cannot be extended.