The short answer: We don't know. The longer answer: Based on personal experience on these forums over the last 10 years, Bungie typically tends to restock them as they approach certain checkpoints throughout the year, holidays, special releases or simply when they catch up on back orders. So, there's no real knowing "exactly when" it will happen. They tend to make some noise on their official social channels when it does happen. It's best to just keep up with their @Bungie handle on twitter and at the official news page bungie.net/en/news for more information.