I've had a Chieftec CTF-650-14cs 650W power supply for quite some time, I don't remember exactly how long but I think for like 5 or 6 years or even longer. During that time it has run a Pentium 4+hd 4850 system for the first few years, briefly a dual core system, an i5 2500k+hd 6870 system for about a year and now i5 2500k+gtx580 since then.
I haven't noticed anything wrong about the PSU but I haven't checked the voltage ratings for quite some time. I'm a bit concerned and I've heard rumors that PSUs might send uncontrollable power surges when failing but also read that they should be protected against such events. Even then I doubt that it's completely foolproof system. So if anyone happens to know more about this I'd love to know, should I buy a new PSU before this one burns out or am I better of using it until my PC won't boot anymore.
6 Years is a great length of time to get out of a power supply, it should last that long, but often they don't. I'd say you could carry on using that power supply for a little while or so. But try to get a new 650W PSU fairly soon, you'll save money in the long term as PSU efficiency has come a long way in the past 6 years, just make sure its a well trusted supplier. It's certainly not worth waiting until your computer doesn't boot as this could be due to your motherboard or another part failing due to a power surge.
Yeah I totally agree with you when you say a PSU should last you 5 - 10 years but he said he's had his PSU at least 5 years, and PSUs have come a long way since then. Like I said it's worth trying to get a little bit more out of it, but why risk your entire system to something that costs 60-70 pounds.