When I go to the Amazon "Today's Deals" page and sort for Kindle ebooks in the listings, the book deals seem to remain the same for about a month. Once in a new month, then they change.
Edited to add that when I go to the Kindle eBook Section and look at Kindle ebook Deals and sort for "cooking, etc" books in the results, those books also remain the same for about a month or so.
The only thing that might show something new would be Lightning Deals which I don't generally look at.