How do I change the duration of the notification messages?

The time duration for messages being shown is 30 seconds by default. You can modify the value in the code if you wish to do so. However you would need some basic development knowledge for this.

You would need to follow the steps detailed in this blog post for the full development process.

In src/scripts/app.js, at around line 430, you should find code that looks like following

$rootScope.hideFloatingMessage = function() {
  $timeout(function() {
    $rootScope.removeFloatingMessage();
  }, 30000);
}

Change the number 30000 to another number you wish. The unit here is milliseconds. After that, follow the blog post above to deploy changes to your production website.