As the others have noted, there are different stability tests for different stages of the overclocking process.
I do this: Once I've settled on some settings, I'll run MemTest86+ first to see if my RAM is capable of running at the HT ("FSB") speed I've got it on. Three runs each of tests 5 and 8 with no errors are usually enough, and will convince me the basic setting is good.
Then I boot into Windows and run OCCT for half an hour (the default test). If there are no errors, I go back into the OS and dial up the settings some more.
I repeat these steps until one of the tests gives an error, which indicates I've gone a little too far with the effort to scale up. I either make a settings adjustment (which adjustment depends on feel and experience; sometimes it's a RAM timings or divider adjustment, sometimes it's a voltage adjustment), or back off of the overclock by a little bit. I then repeat the tests and make sure I get no errors.
Once these settings pass these tests, I proceed onto real stability testing. I run Orthos (Blend) for a minimum of twelve hours, but if I've got time to spare it eases my own mind to run the Blend test for twenty four hours or so.
Speed without stability is useless, in my opinion, so I do my best to make sure my system is stable.
Hope this helps!