They typically dont.
If units are selling well, prices will stay high, meaning no need for sales.
If a new card comes out that replaces/out performs the product, prices may drop for a short period of time (Look at how the 295x2 dropped drastically in price). That being said, holiday sales are common, such as black friday and christmas. That would be the best time to look at it, but dont expect ground breaking sales out of nowhere.