I've read "Published App Feed Refresh??" which implies that updated channel feeds are recognized on a daily basis.  I recently updated my feed to include 5 new videos.  The feed was recognized almost immediately on one of my boxes, but, not others.  I typically rename the feed file each time for two reasons:

1. Amazon caches the file so If I don't name it differently, I'll need to wait till they invalidate the cache.  I'm not sure they'll do that if the contents change without a name change.
2. I like to keep a monthly feed file using the name of the month as part of the name.

Do I need to force an update each time I update my feed with a different name?  Are there any recommendations that take into consideration CDNs that cache their content?