Well cheif...
When I cook using butter and weed, I'll let the butter get a little greenish tint to it before I back it off the heat. Remember now, it's not so much the heat that is causing the THC to be extracted from the weed, it's the fat in the butter.
How hot is too hot? When the weed starts smoking, it's too hot, and at that point, you've probably ruined a good batch of cannibutter; I don't know any other way to tell.