for how long is Content considered "new" by the filter criteria "Content: has new content"? I'm trying to find out how Drupal uses the "has new content" filter criteria.  I'd like to create a View that only shows the latest content on the site, but not sure what is going to be returned.  24 hours, 7 days, 30 days?  

I don't see any way to set the duration, either.
In Drupal 7, the Views module uses the NODE_NEW_LIMIT. If you look at the code used by the views_handler_filter_history_user_timestamp class, which filters the new content, you will notice the following code (in views_handler_filter_history_user_timestamp::query()).

  // Hey, Drupal kills old history, so nodes that haven't been updated
  // since NODE_NEW_LIMIT are bzzzzzzzt outta here!

  $limit = REQUEST_TIME - NODE_NEW_LIMIT;

  $this-&gt;ensure_my_table();
  $field = "$this-&gt;table_alias.$this-&gt;real_field";
  $node = $this-&gt;query-&gt;ensure_table('node', $this-&gt;relationship);

  $clause = '';
  $clause2 = '';
  if (module_exists('comment')) {
    $ncs = $this-&gt;query-&gt;ensure_table('node_comment_statistics', $this-&gt;relationship);
    $clause = ("OR $ncs.last_comment_timestamp &gt; (***CURRENT_TIME*** - $limit)");
    $clause2 = "OR $field &lt; $ncs.last_comment_timestamp";
  }


The same constant is used from node_mark(), which the function that returns a value used to decide if outputting updated, or new in a list of nodes, such as the one shown by the Tracker module.

  if (!isset($cache[$nid])) {
    $cache[$nid] = node_last_viewed($nid);
  }
  if ($cache[$nid] == 0 &amp;&amp; $timestamp &gt; NODE_NEW_LIMIT) {
    return MARK_NEW;
  }
  elseif ($timestamp &gt; $cache[$nid] &amp;&amp; $timestamp &gt; NODE_NEW_LIMIT) {
    return MARK_UPDATED;
  }
  return MARK_READ;


The value of the NODE_NEW_LIMIT is the number of seconds in 30 days.

To notice that when you visit the node, "new" is not anymore shown.