The Cookie Life is the length of time before a cookie will expire on a consmumer's machine, rendering it invalid.  For popular high traffic offers shared by other networks, a shorter cookie life span is suggested to help prevent false positive conversions from tracking due to other networkswhich may have sent a consumer to the Merchant's site during the time that your campaign's cookie was still valid. 

If tracking by Click ID, the Cookie Life span dictates the length of time that Click ID only if the click ID was extracted from a cookie.  If you pass the ClickID to the merchant via the Landing Page URL, then the ClickID lifespan is a rolling 90 days.