If you want to know how long it will run, it can run for a very long time. Right now I have a very old PC with a 6 year old Graphics card and it still runs fine. But because of it's age it can't run almost any game of today.
That being said the better graphics card you have the longer you will actually be able to use it. Getting a high end graphics card will last you about 4-5 years as long as you use it right (OC Correctly etc.) Yours I would give maybe that long if your okay with playing on the lowest graphic settings. If you want it to last longer it's not all about the build, it's about the quality of your GPU.
Keep in mind though that some games (Such as BF4) work your CPU just as hard as your GPU and that might come with future...