I don't think they ever say when a certain size will be back in stock, unfortunately. They only seem to give estimates when a tea is completely out of stock.
I'd imagine the most reliable answer would come straight from Adagio, so maybe try to contact them here: http://www.adagio.com/pages/email.html? ... cc3774c941
...unless someone has an actual email address.
Also, not sure how fast you go through tea, but the black tea should be good for a year or so.