The Radeon HD 7000 series will continue to sell until they are all sold out.
I think the real question is when will AMD stop manufacturing Radeon HD 7000 series GPU cores. The safe assumption is shortly after the HD 8000 series is released.
How soon the remaining stock of Radeon HD 7000's will exist depends on the performance of the HD 8000 series. If the performance is not all that great, then people may simply continue to buy Radeon HD 7000's if the price is good. If the performance is good, then people will buy the HD 8000's and you may see many Radeon HD 7000's for sale on ebay.