Depends on temps as well.
Kinda pointless to run prime if the temps get to 90C (and thus crashes and bombs), and black ops doesn't top 65C, and runs 8 hours stable...
But that's for gamers. People who use their system for other things may need prime 8 hours small fft stable.

I only use these cpu abuse programs for extremely short time periods now, just to make sure I can actaully launch something without corrupting the OS. (except Prime Blend, which is VERY useful for making sure the memory or IMC is working properly).