There was a thread somewhere else exploring how long issues would be kept, but I believe it is 7 issues not 7 days for each periodical. That same thread showed that some periodicals occasionally would keep 8 issues before deleting, and there was a hypothesis about why that may be (it may be the 8 included the current issue).
Personally, I don't worry much once they are in that back issues collection as I just don't go in there and I never have enough files on my Kindle to worry about storage issues.
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