When you enable you book for a free ebook giveaway, you set date parameters for that giveaway.
During that time frame, anyone consumer who discovers the preview will be able to download the giveaway.
The consumer will keep the ebook giveaway; there is not an expiration date on it.
Select the giveaway settings for a book at the time you:
1. Upload an ebook to your inventory. See: How to Upload a New eBook as a Free Giveaway
2. Edit your book settings. See: How to Set-Up an Existing eBook as a Free Giveaway.
Want ideas for getting the word out about your ebook giveaway? Read this article: How to Market My eBook as a Free Giveaway