From http://www.w3schools.com (Copyright Refsnes Data)

RSS <ttl> Element

Definition and Usage

The <ttl> (ttl=time to live) element specifies the number of minutes the feed can stay cached before refreshing it from the source.

Example

<?xml version="1.0" encoding="ISO-8859-1" ?>
<rss version="2.0">

<channel>
  <title>W3Schools Home Page</title>
  <link>http://www.w3schools.com</link>
  <description>Free web building tutorials</description>
  <ttl>60</ttl>
  <item>
    <title>RSS Tutorial</title>
    <link>http://www.w3schools.com/rss</link>
    <description>New RSS tutorial on W3Schools</description>
  </item>
</channel>

</rss>

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Complete RSS Reference