Okay good question.
If your lucky, 6 months. This is because transistors keep getting smaller.
Now the real question is: until you HAVE to upgrade?
Perhaps 1- 1.5 years~. Most games now a days can still be run on older GPU's (1-2 years old). For example Modern Warfare 2 can run on a geforce 6600 (from 2004!), while max settings may need something like a GTX 260 or HD 4870 (hell maybe HD 4850/Geforce 9800).
So really, you should be fine for atleast a year, unless you absolutely must run the latest games (regardless if they suck or not) in high settings.