I think what you most want from a goodreads giveaway is to make people aware of your book's existence, because I don't think it will translate into very many immediate sales. That's why I think it's best to offer one every three or four months. My historical is now on the to-be-read list of 283 people. Thats after three giveaways.

1 for two months- 830 people entered
1 for one month- 492 people entered
1 for two months - 776 people entered.

Don't they say in marketing a person has to see a product at least three times before they might remember it?