this is what i did, i ran 8-10hours of OCCT and was told this was not enough, which i then found out doing prime as it failed after about 4hours.
It does really depend on your stability needs, on how long you check stability for etc. For example, if you're only ever going to use internet and Word, then it simply doesn't have to be as stable as if you're folding or video encoding, because you'll never stress the CPU enough to be affected by the lack of stability. But equally, you might argue that there's no point overclocking to the point at which it's unstable because by definition that means you can't use the full speed of the CPU - you'd be better off having it a couple of hundred MHz lower and then not have to worry about something 'using too much processor' and crashing your computer.
But based on that, if I were overclocking a computer even though I only used it for very minor things, I'd probably be happy with 15mins Prime stable - unless I were using it for 'mission-critical' things - and if it did happen to crash on me, I'd just adjust the settings and perhaps stress-test it a bit more convincingly.
If on the other hand I'm using my computer for video encoding, so CPU is nearly continuously at max, I'd want to very thoroughly stress-test it, probably for 24 hours of Prime blend, small FFTs and large FFTs - stressing memory, CPU, and making sure temperatures stayed OK, because it'd be far more hassle to later discover encoding errors in the videos.
Then again, if I were using my computer for folding, I probably wouldn't bother stress-testing it with Prime for so long - a few hours, perhaps - and then iron out any few remaining stability issues if they arise during folding, until I no longer had any crashes or other errors.