It really depends on each individual CPU and what you're doing with it. There are different defects in each silicon chip so each is going to be a little different from the next.
As far as the AVERAGE temps, I have no idea. I don't own an E6750.
Unless you're using liquid cooling or just have a very cold room, 19-20C is pretty cold for an overclocked processor. I would imagine that anywhere from 25-35C is fine for idle, and anywhere upwards of 65C is fine for load.