Once I’ve clicked on the link in my registration validation email, will I be able to access Pottermore?
We will not be enabling early access to the site straight away. Once The Magical Quill challenge is over we will begin to send Beta registrants Welcome emails as and when they will be able to have full access to the site. We won’t be able to let everyone into the Beta site at the same time, so you may have to wait a number of weeks for your Welcome email to arrive. However, if you have been successful in registering during The Magical Quill challenge, we can confirm you will gain access ahead of the site opening to all in October, so please be patient!

In other words, you might get it tomorrow but you might not. It could take weeks due to how many people there are. :)