In many cases, a longer free period gives your book more opportunities to reach interested readers.
Not every reader opens a newsletter immediately, and personalized recommendations mean readers may discover books at different times. If your ebook remains free for several days, someone who finds it later can still take advantage of the promotion.
A one-day promotion can still be effective, particularly when it is part of a tightly coordinated launch or limited promotional schedule. However, keeping the book free longer generally provides more flexibility for reader discovery.