As has been said, every jurisdiction is different. But a common theme generally applies...
Meet force with equal force, and stop once the threat is neutralized.
Seems pretty standard, but it's quite vague when you truly dissect it and apply it to real world situations. What exactly is equal force? How do you truly determine the threat has been neutralized?
I've heard stories where burglars have been shot, and the home owner went to jail due to excessive force, i.e. the jury was convinced that the shooter's life wasn't in danger. There's also reportedly been instances where someone was attacked, the "victim" defended him/herself and didn't stop once the threat was over, leading to serious injuries to the "attacker," landing the "victim" in jail; such as a person attacks me, I counter, knock them down, and keep stomping their head while they're down, causing permanent brain damage.
Where is the line drawn between equal and excessive force, and when an attack is truly neutralized? Ask a lawyer or police officer. Then try to convince a jury.
Edit: I didn't realize this was in the law enforcement section when I posted it. Have any LEOs chimed in yet?