Solution for Magento 2.1+

Since Magento 2.1 the admin session lifetime is always "session", i.e. until the browser is closed. This has been might have been introduced for security reasons.

The relevant code is in Magento\Backend\Model\Session\AdminConfig:

/**
 * Set session cookie lifetime to session duration
 *
 * @return $this
 */
protected function configureCookieLifetime()
{
    return $this->setCookieLifetime(0);
}

If you want to change this behavior, you can add a plugin for this class with the following interceptor method:

public function beforeSetCookieLifetime()
{
    $lifetime = $this->scopeConfig->getValue(
        \Magento\Framework\Session\Config::XML_PATH_COOKIE_LIFETIME,
        \Magento\Framework\App\Config\ScopeConfigInterface::SCOPE_TYPE_DEFAULT);
    return [$lifetime, \Magento\Framework\Session\Config::COOKIE_LIFETIME_DEFAULT];
}

Where $this->scopeConfig should be an instance of \Magento\Framework\App\Config\ScopeConfigInterface, injected via constructor parameter.

This way the cookie lifetime is used from configuration, just as in the frontend.

Note that the configuration in Stores > Configuration > Advanced > Admin Security > Session Lifetime does not have any effect on the cookies anymore! It is used to determine Redis session lifetime, so if you increase the cookie lifetime, you should also increase this value.

A note here, the accepted answer works however magento 2 uses the default php folder to store session files, if you have this configured in php:

09,39 *     * * *     root   [ -x /usr/lib/php/sessionclean ] && /usr/lib/php/sessionclean

then this is added to the system cron by the php installation.

So then you can either

• disable that cron
• or better: increase gc_maxlifetime in php.ini (as this will be the upper limit)

Another solution without modifing any settings is install any auto refresh addon for your browser and set its time (60 seconds).

It will auto refresh the page for every 6o seconds which prevents session lifetime to expire and we can start working on another tab.

I used Easy Auto Refresh and it works fine for me.

If you are using multiple VMs / servers for web and db and the time is not sync also caused the issue.

So if all options above does not work, check the time stamp / date of the two severs when you are using multiple server for web and db.

I don't know why but for some of us setting the Admin Session Lifetime from backend doesn't work, so I found a couple of solutions:

1. In php.ini change session.gc_maxlifetime from 1440 to whatever number of seconds you desire. (This solution worked for me, tested on magento 2.2.0 and 2.2.1)

2. In .htaccess add "php_value session.gc_maxlifetime 28800" or to whatever number of seconds you desire.

3. By manually setting the lifetime. Go to vendor/magento/module-encryption-key/etc/config.xml and change from 900s to whatever number of seconds you desire.

Hope one of this solutions will work for others too.