Ask yourself this simple question.
At what point does a "simple fist fight" become great bodily harm? Do you know just how much damage you can take or exactly the capabilities of those intent on attacking you?
Put another way, when will the attack cross the threshold from punching and kicking to a serious assault on your person. And at that moment will you then be able to react with deadly force or will you be just one or two more punches away from unconsciousness or possibly brain damage?
The courts have ruled time and time again, its not the actual threat but the perceived threat to the victim. If I come up to you and point my finger at you from inside my coat pocket and tell you I'm gonna shot you and you draw and blast me... they will find no weapon but are you still justified?
The threshold for defense is that you must believe that you are in fear of your life and any reasonable man in that situation would believe just that, absent other circumstances.
Multiple attackers elevates situations in a hurry.
An old Investigator I once worked with years ago, told a man an answer to this very same question, when can I use deadly force?
His answer was, when the thought of what might happen to you is much worse than the thought of going to jail, the time has come to clear leather.