Is it possible to change the valid duration time of the one time login link that is send out by mail to the new user? I think by default it is 24 hours, ist it possible to change this to 72 hours?
In Drupal 8, you need to add the following line to your settings.php file:
$config['user.settings']['password_reset_timeout'] = 259200;
See the documentation for Global overrides:
Drupal 8 retains the possibility of using the global $config overrides. The configuration system integrates these override values via the Drupal\Core\Config\ConfigFactory::get() implementation. When you retrieve a value from configuration, the global $config variable gets a chance to change the returned value.