As I understand it, the RSS feed should be updated every 50 edits. However, much of the time, edits go missing in between updates and are never sent out in the feed. This greatly diminishes the usefulness of the feed.
For example, I just checked two successive feeds and compared the results to Special:Recentchanges. There were about 25 edits missing which were made in the 18 seconds between the lastBuildDate of the first feed and the earliest pubDate of the second feed. This translates to about one third of all edits which never make it to the feed.