Difficult to give a definitive reply, but if you have sown them in pots in the greenhouse, and are aiming to plant out after all frost, they will have a long growing season . By October, there should be quite a large crop of black fruits. While I did not weigh mine, I would have thought about 1/2 to 1lb per plant at least.
There is a page in the soft fruit section, on my website devoted to the Huckleberry.
