Magento 2.1 – Increase Session Time of Admin Panel

protected $_session;

public function __construct(
    \Magento\Framework\Model\Context $context,
    \Magento\Backend\Model\Auth\Session $authSession
) {
    $this->_session = $authSession;

    parent::__construct($context);
}

and then call

$this->_session->isLoggedIn()

Note: don't use objectManager. Its not proper way to do it in Magento 2

Stores > Settings > Configuration > Advanced > Admin > Security > Admin Session Lifetime (seconds)

Also there is a possibility to set this parameter directly to database, just put a value under path

admin/security/session_lifetime

in the table core_config_data

Update

Magento 2.1 admin cookie life time = Stores > Settings > Configuration

Advanced > Admin > Security > Admin Session Lifetime (seconds) or till the user closes the browser

Since Magento 2.1 introduces lifetime for admin cookie as "expires on browser close", together with value in Stores >Settings > Configuration > Advanced > Admin > Security > Admin Session Lifetime (seconds). That means, that session life time equals value in Stores > Settings > Configuration > Advanced > Admin > Security > Admin Session Lifetime (seconds) or when a browser is closed.

Or you can set up a new value for admin cookie, like it is proposed in fschmengler's answer

For Customer Session

Please check this magento user guide related to customer session

https://docs.magento.com/m2/ce/user_guide/customers/customer-online-options.html

I hope this will help