The various doctrines of Libertarianism hold different standpoints on the issue. One must vote for the right man and support the right type of market. But waiting for the right man and the right changes although preferable to bloodshed is certainly not always an option. When the Black Hooded, Jack Booted thugs are kicking down your doors because of your political opinions or because you refuse to relinquish specific rights, it may be too late for politics to save you. Indeed, to say that a man has the right to defend his life against an animal or human predator but not a government that has fallen into a state of madness is to claim a contradiction.
Government is, after all, only men with power.
The question is really one of timing and in places where democracy is less effective such as the two party system in the U.S. of much more relevance. In such places the ballot box has far less potential for change particularly when both parties are almost equally corrupt and totally uninfluenced by the voices of their electorate. Thus the time for revolution may be when the electorate themselves no longer have the power to vote the tyrant out of office or to, in some manner, effect policy from outside of or within the political landscape. But also, let us not be fooled by promises of compromise. Remain ever vigilant and keep your mind alert.