When we talk about apocalypses in RPGs we usually talk about a single, cataclysmic event. It's measured in terms of, at the longest, months, but usually the metric is more modest. Often it's meant to be averted or remembered.
All of which seems wrong to me.
ALL APOCALYPSE, ALL THE TIME
The word, "apocalypse," has christian roots: it's the Greek equivalent of, "revelation," and its usage in this context gestures to the Revelation of John. This, "apocalypse," doesn't describe a random cataclysmic event but is the revelation (revealing, discovering, uncovering) of the true nature of the world. In the christian sense it is the visceral realization of the moral mechanisms undergirding a seemingly unjust universe, the final accounting of eternally saved and eternally damned, the necessity to always be prepared for the thief in the night, to always lock one's shutters against Satan, who prowls like a lion in the outer dark.
In most* professionally published, WoTC/TSR-sanctioned D&D settings, every day lived is a day lived in a perpetual, eschatological drama. Each adventure is often a bit more of the apocalypse received the true nature of things revealed. The literal hands of the gods make landfall on a regular basis, the moral order of the universe is immediately reaffirmed by divine interference and that order is often pragmatically, selfishly ordained. And this appears to be the preferred mode of D&D.
Deities are real and they are horrible, even the nice ones. They fight and kill one another, they induce war among the people of their planet, they attempt to undo all creation, or, in a baffling confusion of roles, require human assistance to thwart the nefarious deeds of their own kin. In their absence they aren't less culpable, just differently culpable. They are the worst, most negligent parent.
Furthermore, they are frequently the beneficiaries, guardians and enablers of a moral order whereby all living, sentient things lower than them on the food chain are subject to long-term punishments, torture in a variety of hells, psycho-spiritual correction facilities and afterlife trash heaps where the "bad people" are dumped. The existence of the soul permits a lost player character a return to life but is also a mechanism whereby everyone not lucky or wealthy enough to be raised is cast into a system of seemingly eternal judgment by gods of no one's election. In most of the popular, published products, this is understood to be the natural way of things; players and GM are happily complicit.
Accordingly, the end game of certain strains of D&D is to achieve god-like power, to break this weird Samara of our own imagining and hopefully kill every single last one of the gods we created together. Pull down the heavens, burn the golden halls and hand and drop the reigns of tyranny.
*Eberron comes to mind as an exception, what others?