Power supplies last forever unless something goes wrong but there are 2 issues:
1. It just plain dies and nothing works.
2. Due to capacitor aging over time, it can supply less power. This will only be relevant if are already running your PSU close to the max power it can output.
Back in high school, my library had just moved up to brand new machines. The science lab had an old windows 95 computer running RD-RAM and a CPU so old it ran with a small heatsink and no fan. It was socket though, not slot. The power supply still worked fine. So did the hard drive actually. A 600mb drive, if I remember correctly. Well, everything in it still worked fine.