That depends on whether you're talking about a press release in a newspaper, or one that's distributed on the web. There a number of web-based free press release companies, but my favorite is PRWeb. It's hooked up to a number of search engines, so with the proper keywords, it will grab it as "news" and distribute it to client webcrawlers who search for page content for those companies with a "news" bar.
For a newspaper release, you want to get it to the newspaper in question at least two weeks before release. For web-based, I tend to put it out as soon as it is available for preorder on Amazon, B&N.com and in the Books in Print update for pre-orders through brick and mortar stores.
One of the things that you want to think about is to try to tie it to something that's already news. Is your book fantasy? Has anything interesting happened in the publishing world in that genre, or with another fantasy author that you could add to the release to tie your book to? I try to make sure that I show keywords on my web based press releases that reference other authors in the same line/imprint as mine. The keywords don't show up on the page, but it gives the crawlers something to look for when they're grabbing links to sort into categories. PRWeb allows you to place your release in more than one category too, although it doesn't appear to do so. But if after submitting the release, you go into the "Edit" feature, you can change the industry/category from Books to any number of other choices, and can release in up to ten categories at once. That really expands your viewership.
Does that help any?
