Well, the best way to use Prime95 is to do a custom Torture Test (go to Options, Torture Test, then click on Custom), and use 2048 Minimum FFT and 4096 Maximum. That way, you are doing roughly 2-3 DAYS worth of the "normal" Blend test in 12 hours' time. If you don't want to do it that way, just run it for at least 24 hours of "Blend" Torture Test, and call it stable if there are no errors and no warnings. If you get either errors (the test will stop if it errors, even once) or warnings, then lower your overclock slightly, or raise your vcore (processor voltage) a notch, and try again.