When does taxation become thievery?

To clarify, I am not an anarchist. I realize that government is necessary and therefore some level of taxation is also necessary. That said, at what point does taxation become out and out thievery; %50, %60, %90? At what point is it thought of as slavery to make a person spend most of their working hours paying the government before they can take any of their own hard earned money home? And before you answer this question heres one more, in what aspect is it fair that the %50 of Americans that pay no income tax get to vote on how much the other %50 must pay?