We usually use something like Prime95 for stress testing and for preliminary testing you need to let it run for 30 minutes to 1 full hour. I personally like RealTemp 3.70 for temps, but some have had issues with it and AMD CPU's. Stress your system for 30 min - 1 hour and monitor the temps. If everything is looking good and you have no issues, you can continue on for a longer amount of time. For a 24/7 stable overclock, I personally recommend 12-24 hours of continuous testing.