Select the Site menu option.
Then down to Global Configuration and click.
Next click on the System menu tab to be able to access the Session Lifetime option.
Now scroll down to Session Settings. Click on the Session Lifetime field and change the value.
The Session Lifetime value is designated in minutes and will automatically logout a user after the number of designated minutes of inactivity.
It is not advisible to set this value too high. Next scroll up and click on the Save & Close button.
You now know how to change the Session Lifetime in Joomla