|It takes a despotic minority too ...|
One well researched answer is "the absence of accountability" (When Accountability Is A Plague, 2, 3, 4).
Another well researched answer is "a long time."
A well known and once-most-quoted historian said it takes social suicide:
"In other words, a society does not ever die 'from natural causes', but always dies from suicide or murder --- and nearly always from the former, as this chapter has shown."(A Study of History, by Arnold J. Toynbee). A well know encyclopedia, looking deeper into that historian's writing, put it this way:
"In the Study Toynbee examined the rise and fall of 26 civilizations in the course of human history, and he concluded that they rose by responding successfully to challenges under the leadership of creative minorities composed of elite leaders. Civilizations declined when their leaders stopped responding creatively, and the civilizations then sank owing to the sins of nationalism, militarism, and the tyranny of a despotic minority. Unlike Spengler in his The Decline of the West, Toynbee did not regard the death of a civilization as inevitable, for it may or may not continue to respond to successive challenges. Unlike Karl Marx, he saw history as shaped by spiritual, not economic forces" ...(Encyclopedia Britannica, emphasis added). So, if we have an absence of accountability combined with a nationalistic, militaristic, and tyranny orientated despotic minority, kiss democracy goodbye.
For example, when a nation's elections eventually are decided by a minority of voters who are persuaded by nationalism, militarism, and tyranny (whether they know it or not), the end is near.
The previous post in this series is here.