Generally if you pass everything you're fully stable. Anything but that it's unstable...
But you need to see. A CPU is only really unstable if things that you actually do make the system crash - and I'm guessing you dont spend every day trying to search for Marsenne Primes...
If you play all your games without any crashes then it's as stable as you need it to be. But obviously you cant really test just playing a 10hour straight run on BF3.
In reality your processor will never be stressed as much as on Prime95 or some others tests, so in some cases even if it isnt stable on prime, your system wont crash.
I suppose it's really what you're comfortable with. If you're rendering HD video and it crashes at 99% completion then you'll be pretty bummed - equally if you're deep in a game, and it crashes without saving then you arent gunna be happy either.
It's up to you really. I personally make sure everything is completely blend test stable for 12 hours - but then again I dont render videos so have nothing to fear from that, and my internet's probably more unstable than my CPU so I'm prettymuch used to that anyway.
All personal preference.