The configuration of the lifetime of first party cookies is embedded in your JavaScript file regardless of the version of your tag (xtcore or SmartTag).
For SmartTag:
You can change it within the application Tag Composer. Choose your site, click on the settings wheel of the "Client-side user ID" plug-in. Finally modify the value of the parameter "Visitor ID Lifetime" (expressed in days).
For xtcore:
Please go to your Analyzer NX interface and then in Tools > Tags > Main Tag and finally in the tab "xtcore.js file (deprecated)". The lifetime is then adjustable via the "Domain cookie lifetime" parameter (always expressed in days).
If you use a third party cookie, then the lifetime of the third party cookie will be 13 fixed months in order to comply with French (CNIL) and European laws. This duration is not configurable.