At the going rate two to three years if intending to the latest games that are released.
But It all depends on how quick the refresh rate of new more advanced Gpu solutions are released by Nvidia and Ati.
That sort of governs what game manufactures can produce in respect to graphics.
DX 12 will be the next reason for most people upgrading to a card that implements the hardware features on the Gpu to support DX 12 instructions.
What it boils down to is what Graphic game settings of a game you are prepared to live with.
As your graphics card ages to be able to play a new title/ game on the market.
It often means you have to turn off a few of the graphics features or FX to make it playable,
with new games.
The choice when to upgrade...