Replacing a PSU is about every 5 years(If you got like a high-end system that has a good PSU like Corsair HX series, it'll last long for about probably 5-10 years)... If you got cheap PSU like 700Watts for $50, you'll need to replace it once every 2 years buy depends how much system used the PSU... More Watts usage for cheap PSU means it'll die faster...
replace it when you either are going to exceed the psu's stats or when your power supply dies.
I have a feeling you are no where near maxing it out, but you must list your system specs for us to be sure on an answer.
Should I start to think about a fresh replacement, or wait for something to break?
You have one of the best quality psu's made. I think I came with a 5 year warranty. Unless you are looking at sli GTX580's, I can't see you pushing it past it's max, perhaps not even then. I would not replace it unless it no longer is strong enough. If it should fail, contact PC P&C for a repair. For what it is worth, they have a very good customer service department.. You actually get to a live person fast.
Noctua NH-U12P SE1366, pointing up to take advantage of the big side intake of the
Cooler Master HAF932, with all it's fans and a Noctua NF-P12 thrown in for good measure
12GB of G.Skill, 7-7-7-21, with this Antec Cool Spot "just in case"
Crucial M225 64GB SSD
Stereo VelociRaptor 300GBs in a RAID 0 off the motherboard
WD Caviar Black 1.5TB
Creative SoundBlaster X-Fi Elite Pro, the kind with the big breakout box and a floppy(!) power patch on the card
EVGA GTX580SC driving the CRT
MSI N450GTS feeding twin Samsung F2380s
Razor Black Widow Ultimate and Mamba (they both light up, see …)
Zip 250 (old die habit hards)
All the cards are bunched together, as I can't get a video card to run in the last MB slot. And since that last wave of drivers seems to keep all the clocks on the video cards up, I think I come close to the "90% usage", as before the MSI never really was asked to do anything while the EVGA was under load.
Under max load the GTX 580 uses about 240W or ~20 amps. and the MSIN450GTS consumes about 116W or ~10 amps. on the 12v rail. Figuring a nominal 10 amps. for the CPU and another 8 amps. to power the other 12v consumers you're at about 48 amps. max 12v draw at max load.
The PCPC 750 Quad has 60 amps. available on the 12v rail so you have plenty of reserve capacity. I have run PCPC PSUs for over ten years without failure so I think you should run it until you need more power as they are fine quality construction PSUs. PCPC and some other PSUs are not as likely to see a drop in performance over time due to use of better components than cheaper PSUs use.