I am doing pre-releases for various ebook titles. Ideally you should use the maximum amount of time (3 months for pre-releases). Why?
It often takes reviewers at least a month to review a title. Publications are more likely to assign and run reviews if they know that the review will run fairly close to the official publication date. If you have a shorter pre-release window for contacting reviewers, you are less likely to have anyone review your titles.
I'm speaking only about ebooks here, but I think it applies somewhat to print titles as well.