if testing for general stability leave priority at 5 and run blend.
If testing to determine max temps at 100% cpu load so see how your cooling is doing use priority 5 and run Gromacs (TAT is better for this kind of test)
As long as you feel like it, depends on how you use your system,
For example when I game (the heavest usage of my machine) I normally do about a 5hr session, so I would run it for 5 hours at least. If you have run a max thermal test for an hour and your temps are good, you then move to stability testing and just let it run overnight.
After all this, the best thing to do is run whatever you run that is the hardest on the computer and run it for more than you normally ever do.
Stable is what is stable for you, If the machine will do what you want it to do for as long as you want it to do with no strange freezes or BSOD imo you are good.
If your computer is monitoring a heart/lung machine, thats different.
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