There is no hard threshold of risk. The warmer the battery gets, the more stress there is on it. Think back to chemistry class. Some reactions occur faster the warmer it is. In this case, the reaction in question is undesireable because you're permanently decreasing the amount of charge the battery can hold the longer you allow the reaction to happen.

That said, I have my setCPU profile throttle down when I hit 45C.

If you want to learn more about Li-ion battery chemistry, google "li-ion plating." And here's a good article showing the effects of temperature on Li-ion batteries:
How to Prolong Lithium-based Batteries - Battery University