When is "enough is enough"?

I think we can all agree that we will never build a perfectly just society. Someone or group will always be at some level, disadvantaged unjustly even if the laws are all perfectly as good as they can be, social inequalities will remain potentially in the form of human imperfections. Humans will always be imperfect when it comes to the sense and practice of justice. Implicit attitudes, even barely perceptible "at the surface" can have big effects on the structure of society as Thomas Schelling and all his influenced subsequent work in the social sciences has shown. Institutions of injustice such as segregation can be de facto established, perpetuated and enforced and surprisingly hard to stop and rectify simply stemming from implicit attitudes that people have (without formal institutions supporting them) and may not even realize they have.

My question is, since there will always be some one or group that may be disadvantaged and the degree of harm done to this person or group may be specific to them in virtue of their basic constitutions such as sensitivity to injustices, when is it acceptable to demand drastic changes to society or even civil disobedience? If it is always acceptable then there will always be the possibility that some group may be permitted to disrupt society even drastically and there will be perpetual civil unrest. This question is obviously related to my previous posts on the relativity of harm and the seemingly impossibility of establishing just political or even social systems. I wish I could find a way to make more clear what I mean by these sets of ideas.