Retrieves the time to live (TTL) from the feed.
[ ttl = ] Feed.Ttl
ttl Integer that receives the TTL value, in minutes.
The property is read-only. The property has no default value.
TTL is an optional feed element. The property value is
0if not specified in the source.
The TTL value indicates how many minutes a feed should be cached before it is refreshed from the source.