I don't know what my review rate is... Many of my giveaway books have just gone out in the mail within the past few weeks, so some readers may not have had time to read and review yet.
But anyhow, before you do any goodreads giveaways, read this first:
How do books get discovered? A guide for publishers and authors who want their books to find an audience
...and then this...
How Readers Discovered a Debut Novel: A Case Study
I've done 7 giveaways now. Some of them have been ten-book giveaways lasting a week, and some have been one-book giveaways lasting only two days. Right now, here's what I think:
1) A goodreads giveaway gets people to put your book on their TO-READ list, which is *great* and looks really nice in your stats.
2) But if your book just sits there on a TO-READ list, that doesn't really do you much good. It's not of any *direct* benefit. You need people to actuallly *read* your book
3) The people who receive a free book from a giveaway are likely to actually read it, and then actually review it.
4) Therefore, you should want to give away lots of books.
5) But you won't improve your TO-READ statistics by giving away 100 books in a single giveaway. In my experience, I get an average of about 50 or 60 new TO-READ adds for every giveaway, regardless of the number of books in the giveaway.
6) A three-day giveaway results in more TO-READ adds than a one-day or two-day giveaway, but anything longer than that has marginal impact.
Right now, my preferred setup is a three-day giveaway with three books, twice a week.
Do that for the first few months of publication, so that you can build up a nice collection of 20 - 30 reviews. Then do a KDP Select promo, and give away thousands of ebooks.
The cost of those free ebooks is much less than the cost of the paperbacks you'll send through goodreads, but I think the goodreads giveaways are a necessary prerequisite for the KDP Select promo.