It should be whatever the person who owns the namespace says it should be.
If you are the owner of the namespace, then you should use a string that nobody else will ever use. The traditional way to make sure of that is to use an HTTP URI that belongs to your organization. For example if Unicef's Canadian subsidiary needed a namespace URI, they would choose one starting with "http://unicef.ca/". It's probably best to use more than just your domain name, as you might in future need to have more than one XML namespace.