Make it easy for users with the High Volume Customer Portal user license to stay logged into your site. Keep users logged in for up to 7 days of inactivity, and allow them to remain logged in even after they close their browser.
Where: This change applies to Lightning Experience and Salesforce Classic in Enterprise, Performance, Unlimited, and Developer editions.
How: In profile session settings, select a timeout value for Session Times Out After, and select Keep users logged in when they close the browser.
Extend User Sessions for High Volume Customer Portal Users (Beta) (salesforce.com)