3 questions, 1 answer.
Enough violence to quickly and effectively stop the person who wants to do violence to you.
Discussion
This could be only a little violence or none at all
Better to be a warrior in a garden than a gardener in a war.
Note that my original quote doesn't glorify violence. It glorifies peacefulness, but sets a higher standard to achieve that glory.
Take a look at the non aggression principle if you are not familiar. This single rule, understood thoroughly, is the foundation of my entire morality. The only trick is to understand it and the situation you face well enough to figure out how to apply it.