Many publishers will specify (in the approval notification email) when to publish your review – for instance, “please hold all reviews until one month before the publication date.” Or they may note a preference in their specific Approval Preferences (on their Browse Publishers page).
If the publisher hasn’t specified, then you’re free to publish your review whenever you’d like, although readers might find it more useful if you wait until the book is closer to publication. Keep in mind that most publicists and marketers want reviews and other coverage of books to happen just before or once the book is on-sale, so that anyone hearing about the book will be able to pre-order or purchase it right away.
If you are ONLY submitting a review or opinion to the publisher through NetGalley (and not posting your review publicly elsewhere), you can do that at any time – but remember that reviews submitted through NetGalley are visible to all members.
Want some tips on writing reviews?
Check out these articles from We Are Bookish, our editorial blog for NetGalley members -- presenting an independent voice to highlight books and a bookish lifestyle!