Prime95

Best way to test ram is to use memtest86+. Run it for a couple of full passes with NO errors.

For prime95, run it untill the cpu temperature reaches it's maximum, and then perhaps an hour should be sufficient.
Specify rounding checking. Again. you should get NO errors.

I really don't think you can benefit from much longer runs.

When overclocking my CPU I tested by running Prime95 for an hour on torture test. You can run longer but an hour should be fine. :-)

I have seen 6-24 hours. Best thing to do imo is let it run, go to bed and work. That is a solid 16+ hours. I have been pushing my 2500k this weekend and I had crashes on the 1.5 hour mark and 2.5 hour mark. (give or take 5 mins)

more time is more stable for system  

In actual use, you will never push the pc that hard for that long.