There are several ways, you can offer your book on LibraryThing, for example. It worked great for me. Or you can announce a giveway on a blog - yours or someone else's.
The duration is up to you, you can make it a week, two weeks, etc. You also choose whether you offer a paper book or a printed copy. As far as I remember, on Goodreads it has to be a printed copy. With LibraryThing, you can do both.Mobashar Qureshi said:A few more questions: what is the best duration for the giveaway? 1 week? 2 weeks? 1 month?