Assuming you're posting from the United States. . .
Let's be realistic here. Even if you don't respond until 20 punches have been thrown you may well get in legal trouble. There are tons of cases in the records which reflect our criminal-favoring legal system. One example was a taxi driver that lost his shirt in a lawsuit. Seems he was sued by the jewel thief who had just robbed a jewelry store. The cabbie pinned the fleeing thief against the wall with his car to keep him from escaping. Another case involved a burglar who fell through a skylight while trying to break into someone's house. He sued because the skylight was inherently unsafe and injured him. He won. Yet another case had a burglar who got himself locked in the store he was trying to rob overnight. He sued because he couldn't get out. He won.
These cases (and there are jillions more) highlight the fact that criminals can file idiotic lawsuits against their victims and win. So if you get attacked, you may well be sued.
That being said, would you rather fight a lawsuit, or be dead? I personally will fight the lawsuit (and the attacker
As to when I will throw the first punch, if it's obvious he's gonna attack me, and there's nothing I can do, I'll probably try to drop him before he gets the opportunity to try to drop me.