When to comment code?

Simple Rule:
We should add all necessary information about code which is not expressed by code itself: reasons we decided to use certain solution, its influence on performance etc.
Ex.:
-why we decided to use in line SQL instead of Hibernate query, etc.
-how the solution impacts performance
-links to business requirements / stories

Ex:

/**
 * Call to this method must be charged - 
 * requirements doc: http://business-requirements/settingsBilling.doc
 * 
 * @param userName
 * @return settings specific for given user.
 *
public Settings getSettings(String userName) {
  Settings result = settingsDAO.getSettings(userName);
  logger.info("Got settings for user: " + userName + ". " + result);
  billingSystem.charge(userName, Operation.GET_SETTINGS);
  return result;

}