Too many variables.
For example, how close is your computer to a heat source? Is the heat on all the time? How good is the air flow around your case? How many devices do have fitted? What's the ambient temperature?
You can see, it's an unanswerable question, however AMD claim the CPU is good to about 85 deg C. But I wouldn't want to run it that hot myself!
To give you some idea, I just checked mine, and keeping in mind the time of year, the CPU is 46 deg C with the internal temperature at 39. (1900 Athlon). So maybe an alarm temp. would be valid at around 60 deg C, but it's up to you!