Question about overclocking a AIW X800XT

I was wondering how long should I run ATI tool on scan for artifacts before I can say im safe with my overclock? I have it at 520 and 560 and 3Dmark05 ran fine the 3-4 times i ran it and even the mother nature test on 3DMark03 ran fine and I was wondering how long should i scan for artifacts with Ati Tool before I can come to the conclusion that im safe? Thanks in advance for the help!