If one of my PSUs die, I just repair it and keep it going. I got PSUs that are 20+ years old that keep on trucking. If it's not capacitor issues, it's usually another passive component like an open resistor, shorted diode, etc.
When the warranty runs out. I always over provision for my needs so it's never been capacity issues. Generally the replacement has improvements that help make it worth it, like quieter operation or no more electrical noise when opening documents (looking at you, Seasonic 1050W).
Only if it goes bad or my power demands become higher than what it's capable of delivering reliably. I don't really do "preventative" swaps. Often times, even when i'm doing a completely new build, I will carry over my old PSU if it has the capacity, unless its 7+ years old.
I don't really replace mine. More so cycle them through my primary and back up rigs. I have a bit of an issue with buying power supplies when i see a sweet deal. As a result I have a shelf full of Seasonic, Superflower and EVGA units waiting for client builds (right now is a great time to be a builder, easyyy money).
I always go back to one of my two favorite beasts tho (love those custom cables) evga g2 1300 or Seasonic PX 850.
I almost bought another Superflower leadex 3 850w gold from neweggs shellshocker email yesterday but forgot about it and missed out. Itll be back around They go on sale regularly and are really solid units if your looking. There's a 750w unit on sale today but its kind of weak as its the same price as the 850w was yesterday @ $109.
Something of note with these SF units. They now advertise "Dual Over Power Protection" on the product page lol. Someone was paying attention and wants to capitalize on the gigabyte debacle!
I only replace a power supply if the old one dies or I need more power. I usually buy a new power supply when building a new system because I keep the old system in use so there's no power supply to transfer over.
As long as the PSU is strong enough to power the system and has no reliability issues it stays in use. My server is using a 650w PSU I bought around 14 years ago and it's still going strong. I wouldn't be using it in a system with power requirements anywhere near the total power of the PSU as PSUs do degrade over time but the max power requirements for the system right now are probably well under half the PSUs rated max load.
I always have a spare power supply to keep around for troubleshooting. You can save a heap of chasing your tail during unknown troubleshooting times. Replaced mine after 10 years & it was still working fine but did it preemptively to not suffer with a dead power supply if it were to fail anytime after 10 years.
My trick is to get a power supply that is 3 times my max power usage and put a dedicated fan on it to keep the components at ambient temps where the components are rated to last the longest. Had my 1200w Corsair unit since 2013 and still going strong. It’s an AXi unit so I can real time monitor the voltages and it’s still dead on 8 years later.
I've had a few conk out shortly after installation, but most PSUs I've had last 6-7 years -- and really that's been me simply replacing them due to lack of warranty or new requirements. Like a good wife/husband/partner, if you find a good one, hold on to it
Every 3 years or so I measure the voltages and ripple of the 5V and 12V rails. If they start to exceed the ATX ripple tolerance, I'll perform capacitor replacements. That said, I have several PC and Amiga power supplies that are over 30 years old and run perfect like the day they were new from the factory. Spending less than $20 on capacitors is far less than buying a new power supply.
IMHO, in most cases (e.g. not Gigabyte), as long as you feed it good clean power, it should be one of the longest lasting components you own. I do emphasize good clean power though. So an intervening UPS is recommended (almost any should suffice). Voltage fluctuations (sags) can really shorten the lifespan of a PSU.