I agree with Mike that 7 days is a good time to announce your affiliate toolbox to your affiliates. I'd say 10 days is fine too.
It's also a good idea to launch your website several days before launching your actual product, and invite your affiliates to let people know about this pre-launch site.
The key is making your pre-launch website interesting somehow. For example you can give away free gifts to people (like related ebooks), or a poll that invites people to participate and so makes your site interactive.
You can also add a countdown script to your home page to build anticipation about your launch.
Just make sure you tell you affiliates that they will get credit for visitors they send to your pre-launch website too (by simply adding a cookie to the visitor's computer and tracking later purchases).
By the way, what you've decided to do (letting your affiliates know about your product beforehand) is a fantastic idea, Jennifer. Because not only it lets them set their schedule to be able to promote your product when it's launched, it will also make them feel special because you're letting them know about your new product before everyone else finds out.