So long as people see things as better now than they used to be, they will continue to support the system indefinitely. Only when the system loses its momentum

So long as people see things as better now than they used to be, they will continue to support the system indefinitely. Only when the system loses its momentum do people start thinking they may be better off without it. A generation after the system stagnates, young adults start protesting but the bulk of society stays the course. You see the emergence of countercultures but not much real change. Often progress restarts just by chance and the society recovers. Two generations of stagnation and now the bulk of society wants change, but the ruling generation is still contains many true believers. There is an obvious clamor for reform, which is met either with half hearted efforts or suppression. Such a society may endure a passing crisis, but usually it needs a strong kick in the butt to actually fix its systemic issues. Three generations of stagnation and the disaffected are now in power, though true believers are still alive. Much more serious efforts at reform are made. However the oldest still hesitate, while the younger generations no longer see what's left to preserve. This is the last chance to turn things around, though more often than not there is political looting and kleptocracy as people start to doubt the system can be made to work again. Four generations after stagnation there is no one left who wants to preserve the system, while those who want to go further than just reform are starting to enter power. The question is not if it will collapse, but when, and how violently. Typically if the society has maintained its plateau, it will slowly but mostly painlessly dissolve as the central power becomes irrelevant; if there is a sharp decline in the midst of a crisis then typically there will be a sudden series of relatively clean breaks; if decline has been slow and steady then the empire will violently rip itself apart in power struggles. Typically some successor arises which is a nominal continuation of the society but has no realistic path to recovering its former glory, and any renaissance is really a new society seeking to use its connection to lend it legitimacy. The trend is consistent but generation length is variable. In societies with slow communication and diverse populations, a cultural generation may encompass many biological ones. On the other hand for a fast communicating and relatively homogenous society, it may be less.