Here's the rule from the website. The expiration date depends on when you join, and you should have 30 days from the end of your membership year to redeem. If you look at "my account" on the erewards web page it will list your expiration date, and the dollar amount subject to the expiration.

Section 8) Expiration of e-Rewards Currency

All Members of e-Rewards will be assigned a Membership Year based upon the date of enrollment in the e-Rewards program ("Membership Year"). As described below, Member's initial Membership Year will be between 12 and 15 months in duration. Each subsequent Membership Year will be 12 months in duration. Each Membership Year will end on the last day of a calendar quarter and shall be determined according to the Member’s enrollment date as follows:
Members whose enrollment date is on or before December 31, 2002 shall be assigned a Membership Year ending on June 30 of the year following enrollment and June 30 of each year thereafter.
Members whose enrollment date is on or after January 1, 2003 shall be assigned a Membership Year ending on the last day of the calendar quarter in which the Member enrolled of the year following enrollment and on the last day of the calendar quarter of each year thereafter. For example:
Those who enroll between January 1 and March 31, 2007 shall be assigned a Membership Year ending on March 31, 2008 and then on March 31 of each year thereafter.
Those who enroll between April 1 and June 30, 2007 shall be assigned a Membership Year ending June 30, 2008 and then on June 30 of each year thereafter.
Those who enroll between July 1 and September 30, 2007 shall be assigned a Membership Year ending September 30, 2008 and then on September 30 of each year thereafter.
Those who enroll between October 1 and December 31, 2007 shall be assigned a Membership Year ending December 31, 2008 and then on December 31 of each year thereafter.
All e-Rewards Currency earned during any Membership Year must be redeemed within 30 days following the end of the Membership Year. (For example: A Member who enrolls on August 21, 2006 is assigned a Membership Year ending on September 30, 2007 and must redeem Currency earned during the Membership Year running from August 21, 2006 to September 30, 2007 by no later than October 30, 2007.)
Following the end of the 30 day period, as set forth in Section 8.b. above, seventy-five percent (75%) of any unused e-Rewards Currency remaining in a Member's account that was earned during the just-ended Membership Year will expire, and will be removed from the Member's account. The remaining twenty-five percent (25%) of the e-Rewards Currency earned by the Member during the past Membership Year shall be automatically transferred into such Member's account balance for the next Membership Year. For example: At midnight of July 30, 2007, Member has $100 of unredeemed e-Rewards Currency earned in the Membership Year ending June 30, 2007, and $5 of unredeemed e-Rewards Currency earned between July 1 and July 30 2007, for a total account balance of $105 in e-Rewards Currency. On the morning of July 31, 2007, Member will have a total account balance of $30 in e-Rewards Currency: $5 earned in July 2007, plus $25 (25% of $100) rolled over from the Membership Year ending June 30, 2007.
At e-Rewards' sole discretion, e-Rewards may choose to waive or defer enforcement of the expiration of Member’s e-Rewards Currency.