It depends on the PSU and much work it has to do. From day one a PSU will be slowly deteriorating, its efficiency and the quality and amount of current it can supply will all fall. If you have a quality overpowered PSU then it will work for ages, but if you use a poor quality PSU that sits on borderline of being capable of what is required, then the deterioration will lead it to fail far sooner. I buy overpowered quality PSU's and more or less all have lasted 10 years, at which time I retire them to super light non critical duties, but I have also seen many cheaper PSU's die in a handful of years.