The official B&N "
Free eBooks" page seems to be updated roughly once a week, when they do the Free Friday promo and they seem to have stopped bothering to add in the other publisher promotional free books in the B&N store which have shown up in the meantime.
Your best bet for finding free Nook-compatible books is to check the Deals, Freebies, and Resources forum here, and also the B&N Bookclubs for Nook on their site. How long a promo lasts depends on the publisher, and it seems to be generally to be 1 week on average, although some deals disappear pretty fast, and if you see something you want, don't hesitate to jump on it immediately. It might not be free anymore the next time you look.
You probably already know this, but in case you didn't, as a Nook user, you can also use the freebies from the Sony and Kobo stores, as well as anything that supports the Adobe Digital Editions DRM. It's also worth checking out the Kindle listings to see if someone's added a note that the book is now free in another store.
Hope this helps.