This is hard to discover these days. We used to have a nice document from an expert on the subject of temps, but no more.
First up, a CPU has many different temps. The temp usually displayed in your BIOS is called Tcase, and it's from a sensor in the case of the CPU and not very useful.
The useful temps are the 4 core temps. These can be discovered using the program Real Temp, Core Temp, or HWMonitor.
Keep in mind that all such programs are calibrated. The sensors do not report actual numbers... they send data which then must be interpreted and different programs can interpret temps differently. So, you might see two programs reporting temps that are 5 or ten degrees apart. Yes, it's usually 5 degrees or ten, not 6 or 9.
So, we use one of the above core temp programs because they all tend to agree and so it's standardized, see?
My impression of core temps in i7:
80c too hot
60c warm but safe
50c and below is cool.
So probably low 70s under Prime95 is OK, as long as you don't make a habit of pushing the CPU that hard normally.
I have heard that throttling only takes place at much higher temps though, which would indicate, if true, that Intel thinks 80c or higher to be OK.
HOWEVER... a Sandy Bridge processor is not likely to get that hot if properly cooled. It will get unstable before it overheats. Provided you are not trying to OC with the stock cooler or improperly applied thermal paste.
During stress testing with a Cooler Master Hyper 212+ (push-pull), my i7-930 (stock) never reached above 52C (highest core temp @ 100% load). As long as you have good airflow and a solid cooler, you should be under 60C at load at stock settings. OCed is a different story since you're pushing more voltage (usually).
100C is a bad thing which will activate protection circuits.
68-72 is considered acceptable under stress testing
I'm typing from my son's 920 running at a every day (24/7) 143 % OC (3.8GHz) and temps run 55-58 under P95 w/ a Prolimatech Megahalems. It's substantially undervolted at 1.1125 volts. With the Rampage MoBo, it has multiple BIOS profiles which run up to 4.4 GHz. At that point, P95 temps reach 69-73