Only a few minutes (5-10) will give you top temp, or within a few degrees of it. Any more is just testing for stability. You have plenty of temperature room to overclock if you so choose. Generally, the max load temp is 25c from TjunctionMax (yours is 100c), so 75c is the maximum you want to let it get. For normal use, I try to keep mine <60c because I fold @ home 24/7.