More like at least a year, based on one of my purchases. I still can't understand why they can't go back to a single wishlist that is synced between the site and your devices. But then, I'm not a software engineer.
I've had this issue now too. I had a book on my Wishlist, purchased the book, and now I can't remove it from my Wishlist, and B&N aren't doing much to fix the issue. I'm really frustrated with this. The only solution I can see is just not to be anything on your Wishlist, because if you buy it then you won't be able to remove it later. My advice. Put things on your Wishlist, but before you buy them, remove them, and then buy them through the website, rather than through the Wishlist. That is what I'm doing now and it seems the only solution, until B&N fix the "bug"