The cookie expiration date is one year after the date the cookie is created.  With metering there is a 30 day limit hard-coded that resets the limits on how many assets can be viewed in that time period.

The URL custom property metered_force_expiration_date can be set to force an expiration date for the metered cookies.  The default is null (they never will expire). The acceptable values/formats for this property are:  null, "YYYY MM DD", "DD YYYY", "M DD YYYY", or "M/DD/YYYY".  The force expiration option is a method to kill old stale cookies.  It takes a date as a setting and will kill any cookies that were created before that time. Users with stale cookies will be treated as if they had never visited the site before and given a fresh batch of cookies.