This excerpt taken from the AMZN 8-K filed Sep 18, 2007.
2.11.2 Delivery to Corporation; Duration
A proxy shall be filed with the Secretary before or at the time of the meeting or the delivery to the corporation of the consent to corporate action in writing. A proxy shall become invalid three years after the date of its execution unless otherwise provided in the proxy. A proxy with respect to a specified meeting shall entitle the holder thereof to vote at any reconvened meeting following adjournment of such meeting but shall not be valid after the final adjournment thereof.