Multiple days in a row, in fact possibly all 5 days at once. You can't get enough traction if you do a single day. You need multiple thousands of downloads (preferably tens of thousands) in order to see a bounce when the book returns to paid.
Also, it takes a while for your rnaking to be recalculated when it goes from paid to free, and then takes a little while longer for you to appear on free bestseller lists (which is where the real heavy lifting happens -- many people only "shop" from those lists for their freebies, so if you hit them then your downloads will go up exponentially but if you don't hit them then they won't -- it's almost as if you don't exist).
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