There are a large number of factors that effect PSU life span. If the manufacture uses cheap parts then it won't last as long. If your system runs really hot and the manufacture keeps the PSU fan at too low a speed then it will shorten the life. Also if your pushing the PSU close to its limits that will wear it out faster.
Since Tagan is a pretty good brand and if all other things are average then you should get 3 to 5 years out of it. Unless something goes wrong like a power surge or there is a defect in the unit. But you can say 3 to 5 years is about the average for most decent quality PSUs. I recommend you don't use a PSU for longer then 4 years. Since time is also a killer of PSUs. Its mainly the capacitors that wear out over time. The electrolitic inside them dries out from heat and over time thus reducing there effectiveness.