For me, it would depend on the current weather conditions, and the attitude of the animal I was thinking of shooting.
Obviously, bad weather would shorten my comfortable range, as would the condition of the animal. If an animal appeared excited, or anything except completely relaxed, I don`t like to take a shot.
I know you probably want to hear a number, and I don`t have one. I can safely say that on a practice range, I can hit a 5" bull 10 out of 10 times with broadheads at 50yds.
Would I shoot a deer sized animal at that distance? I won`t say never, but I sure like those 15 yd shots better.