i agree with the other replies, up to 5 years if you bought something in the upper tier and are happy to potentially reduce settings/resolution as more demanding applications come out. I gave my daughter an old intel q8400 pc, with an 8500gt vido card, 4gb ram and windows xp, and it is still of some use. She mainly plays minecraft on it, and some GBA/n64 emulator games.. So yeah, depends what your expectations are.
My general theory, is to get more cpu power than you need, and sacrifice on the vid card a bit for a gaming system if your on a budget. Cpu sockets become outdated quickly and cpu performance is stagnated more-so than gpu. Where gpu's are easily upgradeable and newer pcie versions will be backwards compatible with older...