A VERY subjective question requires a VERY subjective answer. I remember hearing someone once saying that they didn't understand why, when they were in school, kids picked on each other. Taking advantage of each other seems to be a common theme among humans in general. In capitalism, it's the rich influencing the government with money and perks to take advantage of the market (AKA the consumers AKA the general public and possibly AKA poor people). In socialism, it's the "poor" and middle class influencing the government with their vote (AKA putting politicians into power) to take advantage of the rich, even if the rich have worked their asses off to get where they are.
So my answer would be - when the people being taken advantage of have had enough and are willing to take the risk of combating their oppressors. That can be several levels as well though: Complaining about it, formally complaining, taking legal action, or taking vigilante action. A lot of the time we try to separate (or maybe I try to separate) it into "You either don't do anything or you bomb the place"... however, there is a mass gray area. I've also heard before that "people deserve the abuse that they are willing to take from others." As much as I get kind of puzzled with a "That's not right!" feeling at the mention of that notion, it may be the way that the world just is... and we just have to accept it at times and act accordingly.
