It also depends on how 'clean' your power is when it comes out of the wall, because the power you are getting is not constant but has surges and other noise in it, and those tend to put additional strain on your equipment and if your PSU is no good will pass that noise to the rest of the components. A UPS typically does a lot to condition power and so it will tend to make your PSU last longer, keeping the PSU cooler and less humid will also extend life, etc. Also running at less than full rated load is easier on the power supply since all the parts are rated for more than that, up until a certain point where the chronic underload becomes more of a problem.
In short the lifetime is very hard to predict and it's influenced by a lot of factors.
If you expect to use power in the 400-500W range you probably want a power supply in the 600-800W range, or a very high quality one. You want at least a 20% margin but you also don't want a situation at where you'll frequently be bellow 30% load (for efficiency reasons, it is at this point that most power supplies will rapidly lose efficiency), though good power supplies should be fine until you start hitting the 20% or less load consistently.
E: "Did they... did they just endorse the combination of the JSDF and US Army by showing them as two lesbian lolicons moving in together and holding hands and talking about how 'intimate' they were?"
B: "Have you forgotten so soon? They're phasing out Don't Ask, Don't Tell."
In short the lifetime is very hard to predict and it's influenced by a lot of factors.
If you expect to use power in the 400-500W range you probably want a power supply in the 600-800W range, or a very high quality one. You want at least a 20% margin but you also don't want a situation at where you'll frequently be bellow 30% load (for efficiency reasons, it is at this point that most power supplies will rapidly lose efficiency), though good power supplies should be fine until you start hitting the 20% or less load consistently.
E: "Did they... did they just endorse the combination of the JSDF and US Army by showing them as two lesbian lolicons moving in together and holding hands and talking about how 'intimate' they were?"
B: "Have you forgotten so soon? They're phasing out Don't Ask, Don't Tell."