Antimetrics :
I know but I mean like do they release the top end graphics cards every two years?
Basically they announce a graphics card product line (series, generation, call it what you wish). Like the 7xxx series. Then it really varies after that. Sometimes the top end cards release right away but sometimes they have yield issues in production so they release middle cards first and the top end cards trickle in over the next month or two.
Generally speaking every 18 months we tend to see Nvidia and AMD release a new generation of cards. This time around will be more unusual since both manufacturers have decided to delay their next series of cards. So we'll probably end up going close to 24 months without seeing a new generation.