You can change it within the application Tag Composer. Choose your site, choose your configuration and then edit the settings of the "Client-side user ID" plug-in. Finally modify the value of the parameter "Visitor ID Lifetime" (expressed in days).
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.