Authenticated User Session time length.

Have you set up your Sessions Database driver?

If so, you can query the timestamps on the sessions table.

Or (and this is by no means the best way to go about it, but it may point you in the right direction) you could set up your own table and capture the last time the User logged in like so:

Event::listen('auth.login', function($user) {
    $user->last_login = Carbon::now(); // assuming you place a last_login property on your User model

    $user->save();
});