Just enter your Hop Link landing page URL followed by a space and then the number of days your affiliates' referral cookie should be valid for -- e.g., for 3 years (1095 days):
http://www.example.com/landingpage.html 1095
Note that this is specific to each hop link you offer, so just setting up your Common Hop Link like this won't automatically apply the custom cookie duration to any product-specific hop links you may also have set up; you'd have to set those up with a custom cookie duration of their own.