Mid range games at high-max settings at 1080p with reasonably high performance (means except the most intensive games etc) you will probably do fine for 1.5+- years at stock, perhaps a bit longer. Once you overclock everything you will probably take it to 2 years or so. After 2.5-3 years you will need to buy a new rig. In fact, the moment the next gen consoles are out, your performance will drop by more than half compared to what you get in current gen games.
So sometime in 2014, and if you really push it then sometime in 2015. At most.
If money is an issue, then get Core i5 2500k (MC) or 3570k plus a mid range mobo plus 8gb ddr3 1600 ram plus 2gb 7850 and oc everything. Will last 1-1.5 years comfortably. Half your money or nearly is saved. Sell this and buy a new system in 2014 with the money 🙂