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So quick answer.. its the publishers decision on how often to update. It your decision on how often to veiw the latest feed depending on your RSS reader settings.
If it is a site that is aggregating RSS feeds they usually have RSS cacheing, or set their own intervals (say once an hour or once every 15 minutes to see whether a RSS file has changed).
The homepage feed is updated whenever the homepage is updated.
The active list one is updated every 15 mins by a cron job (same one that sends out emails). Any faster than that, and it is mostly duplcate stories anyway. I felt 15 mins was enough.
The homepage one is linked from the homepage on the lower left menu and the active list one is linked from the bottom of the active list itself.
There is also an IFRAME option [webmasterworld.com] available for those that want to put the active list on their site.