In general, if you're looking to get requests and early reviews well before the pub date, then we recommend including books in Promotions earlier rather than later to give members plenty of time to request, download, and read—especially since our inventory fills up very quickly.
That said, we often have publishers include books in Promotions even when it's closer to the pub date as a sort of "last chance" for downloads/reviews before on-sale. Or we can use a Pre-Order or Buy Now CTA (linking to retail) if you'd prefer to drive that activity (instead of NetGalley requests/downloads) from the Promotion.
It's really up to you and your campaign strategy, and whatever timing you're most comfortable with in terms of when you'll be able to manage the resulting requests. (But if your book has Read Now availability, then you don't have to manually approve/decline requests!)
And the good news is that you can reserve Promotions as far in advance as you'd like, even if you need to indicate "TBD" for certain fields on the booking form. We highly recommend taking a look at the schedules in our Media Kit and submitting your bookings as early as possible to secure your spots!