I set the book free a day in advance in case there are any complications with the price change. The Bookbub hits on day 2, the lift stays high on day 3, and gently drifts downward on days 4 and 5. I used to stack a few ads around the Bookbub to prolong the effect, but other advertisers have become so weak for me these days that I don't bother stacking anymore.
So, minimum of three days for best effect. No harm in taking the extra 2 as well, if you have no plans to use them elsewhere.
Small sample size, but I did a 3-day first and sold 1800 copies of one book. A year later I did a 5-day of the same book and sold 2200 copies. So not much difference per day, but I stayed sticky at about #7000 for months on the second. The first drifted within a couple weeks to about 20k before sticking.
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