By default, FeedBurner will check your RSS feeds every half hour. Depending on the content of your site, however, you may want your RSS feed to be updated instantly. FeedBurner allows you to visit a page to re-ping your feed and update, and you can use this feature to have your feed updated automatically.
All you have to do is place the following URL in Settings -> Writing -> Update Services:
http://feedburner.google.com/fb/a/pingSubmit?bloglink=http://www.domain.com (be sure to update “www.domain.com” with your site’s URL)
Save the settings, and now WordPress will automatically send a notification to Feedburner when you publish a new post, which will instantly update your RSS feed.