It could take quite awhile for the heat to saturate the heatsink. The heatsink needs to be as cool as possible to keep temps down. Huge metal fins + big fan blowing cold air over them, isn't going to warm up in a hurry!
I don't like going over 70C myself, but opinions vary. If I was at that temp I'd try reseating the heatsink with some Arctic Silver 5, if that didn't work, I'd look for better cooling options (higher ranking air cooler or a water cooler). Easiest and cheapest would be just to back off the overclock/voltage a bit.
I imagine it'd get hotter if you ran the Prime95 torture test on Small FFT mode. I'd try that, using a program to observe the core temperatures as it runs. Stop if it breaks 80C.