There's not a magic line that you cross from detail to no detail, partly because faces have a range of highlights on them.
If you are talking about how much you can overexpose a face and bring it back down to normal without it looking odd, I'd stick to 3-stops over as the limit for color negative, 4-stops is pushing it but possible with Vision-3. The new Vision-3 stocks seem to have another stop of overexposure detail that can be pulled out.
But I can't give you an exact number. The old rule was that a caucasian face had to be 5-stops overexposed to burn out to white, but maybe that's more like 6-stops now.
However, most HD cameras don't allow more than a 1-stop overexposure before you start losing detail in the brightest parts of the face. 2-stops over is possible... but not recommended. Generally a caucasian face that is 4-stops overexposed is burned out more or less, though on a RAW or LOG recording, this may not be true because often RAW or LOG recordings are a bit on the dark side with Zone 10 on a grey scale not hitting 100 IRE, in order to retain more detail in bright areas.
