You can change the cookie lifetime directly within the JavaScript file.
For SmarTag
Go to Explorer > Data Quality > Data Collection Portal > Tagging > Tag Composer.
Edit the settings for identified visitors and change the value for identified visitors data lifetime. The lifetime is in days.
Once the new version of the file is downloaded, you need to implement it into your site so that the updates are taken into account.
For Piano-analytics.js
You can change it by setting a new value for the configuration key storageLifetimeVisitor: Javascript | Piano Analytics Developers Documentation (atinternet-solutions.com)