i think overall, you have to know what your stress testing FOR and WHY. If your clocking your system, and just want to see if its stable, then a few min in prime/memtest should do that for you.. all you need to know if if your cooling is going to keep you within tolerances, want to see if your over/under volting is going to BSOD your system when it does some real work, or if your memory will handle an odd BClock setting.
However, I have found stress testing a system for hours valuable for working out some hardware faults.. now, read that again, i said HARDWARE FAULTS
Ive had random BSOD on a prior system, for the life of me couldn't figure it out. Didn't even overclock it.. Prime showed me heat wasn't an issue.. temp probes said everything was ok, and memcheck passed after an hour still, so couldn't have been memory.. well actually it WAS memory.. I got fed up and let it run overnight to "make sure", and sure enough, when I woke up 4 hours into a memcheck, I hit a whole bank of errors.. i thought it was probably just a fluke it took that long.. ran it for an hour.. nothing.. ran it for another hour another day.. nothing.. wanted to see if it would do it overnight again, sure enough 4 hours in.. bam.. errors galore. RMAd the ram, ran the system overnight again.. 100% passed, and never got another BSOD on that system over the next 8 months i owned it.
I also agree with Papasmurf, that if you have a perfectly good running system at stock speeds, and are just OC a step or two, a 30 min prime test is going to be just as good as a 6 hour one.. (i give it 30 min, because that's generally how long it takes to run all 10 cycles of testing types on the CPU). If the system is going to crap out on an OC, it will do it when your CPU spikes to 100% load and starts sucking up power.. and that happens right away when you use programs like prime.
memory same thing.. you over/undervolt your memory too much, or run too high of a ratio on it, memtest is going to start showing errors in a few minutes not hours..
Errors that crop up on an OC'd system randomly like BSOD while playing games and such, generally comes from other aspects of the OC, not because the system wasn't left to stress test over night. For example, I was JUST having problems with BSOD randomly in games.. memtest was fine, prime ran fine, cooling is fine.. everything was fine, I chalked it up to "must be Nvidia's drivers" because it was ONLY in games..
Turns out, after getting fed up and reading more, I just needed up up my IOH voltage to 1.35 from what it was to give my SLI setup a little more juice.. since I did that, everything's smooth and no more BSOD.. I could have run prime for days on end, and that still wouldn't have shown up and my system would have looked "stable"..Edited by Telnets - 1/29/11 at 2:28am