For Ubuntu Server php.ini is located at /etc/php5/apache2/php.ini
sudo nano /etc/phpX/apache2/php.ini
Simply change the default value of 1440 to your desired value for session.gc_maxlifetime increase duration of session
#session.gc_maxlifetime = 1440
session.gc_maxlifetime = 7200
Restart apache service to apply change
sudo service apache2 restart
You could verify by calling below page
<?php
echo ini_get("session.gc_maxlifetime");
?>