My training was to fire UNTIL the treat has stopped and then stop firing. I've heard people say to fire two shots, reassess, fire two more if needed, reassess, etc.

Realistically though, if you're firing a weapon at a person who is trying to kill you, you are feeling a LOT of fear and adrenaline. It's very difficult to think clearly and logically under those conditions. You're probably going to just fire until the threat is clearly over, and you're probably going to be able to get many rounds fired before the person making the threat is down.