While reading about the Duskfall, it struck me that the placement of the event in time is very vague.
The earliest upper boundary with evidence is Middle Merethic Era, when Topal the Pilot explored southern Tamriel. Although he made contact with argonians, the book Father of the Niben does not mention them having a noteworthy civilization.
>The fetid, evil swamp lands and their human lizards
Retreated to the east
-Father Of The Niben, Fragment Three
If Topal had arrived before the Duskfall, I find it unlikely that he would characterize the swamp lands as evil and fetid without at least mentioning the civilization. From this we infer that the Duskfall happened earlier than Topal's journey.
Of course, it can be claimed that many verses describing Topal's journey are lost, and do not explicitly note the absence of advanced argonian civilization.
Another, later boundary can be set at 198 First Era, when Barsaebic Ayleids arrived into Blackmarsh, and could not believe the argonians to be capable of constructing the xanmeers.
>The older ruins are surprisingly complex. Lizards never built these."
-Ayleid Scholar, 198 First Era
Ayleids are elves, and hence very long lived. Additionally, they likely did not read of pre-Duskfall argonians, because they did not think them capable of advanced construction. This places the Duskfall likely before ayleids appeared in Cyrodiil, reiterating the boundary of Middle Merethic Era.
Topal's expedition seems to be the earliest known account of post-Duskfall argonians, but I believe that for a more adequate dating we must look into what event could have caused the Duskfall, as there are almost no sources on Tamriel prior to elvish colonization.
While researching the subject, I stumbled upon this post. I generally agree with the Duskfall happening during the Dawn Era, and will try to further analyze this interpretation.
It is established that the Hist and changing view of Sithis played an important role in that event. It seems implausible that this dramatic reversal of theological views(both among the Hist and therefore argonians) happened without an important metaphysical cause. On Mundus, earthly changes reflect changes of the divine, therefore to understand the Duskfall, we need to pinpoint what happened to the corresponding deity – Sithis.
Sithis is the soul of Padomay, the force of change. Lorkhan represents change as creation, while the god of destructive change is Mehrunes Dagon. During the Convention, Lorkhan was severely weakened, his heart being ripped out and body becoming Masser and Secunda.
Now let us look into the implications of this event for the Saxhleel and the Hist. The argonians were created after/during the Ehlnofey War. They worshipped their creators, the Hist, and Padomay. Initially Sithis contained both creative and destructive aspects of change(Lorkhan and Mehrunes Dagon). But at the Convention and "murder" of Lorkhan, the creative side of Sithis was damaged, while its destructive side remained intact.
This led to Sithis overall becoming a god of destruction more than one of creation. And as their god turned more destructive, so did the Hist and Argonians, abandoning their stone nests.
These considerations create a quite narrow time window for the Xanmeer Builder Civilization: Dawn Age, after the beginning of the Ehlnofey War, but before the Convention.
This way of thought also explains why the An-Xileel are returning to the Xanmeers: during Oblivion Crisis Mehrunes Dagon was defeated(both by Martin Septim in Cyrodiil and by the Hist in Black Marsh) and the destructive aspect of Sithis was weakened, while Lorkhan was better than in a long time, his heart liberated and Talos hypothetically taking his place.
Sithis became a more creative deity, and the argonians returned to the pyramids.
TL;DR: The Duskfall likely happened in the Dawn Era as a result of Lorkhan's destruction that lead to Sithis becoming more destructive. Defeat of Mehrunes Dagon in the Oblivion Crisis led to reversal of this event and return to the xanmeers.