Eramis and Taniks were both alive back during the Fallen's Golden Age, and both seemed to still be in their physical prime when we killed them (Taniks before he lost is original body).
Given that the Fallen's Golden Age was at least several centuries ago, this can only mean the Fallen have an extremely long lifespan.
Awoken live a long time, but goddamn the Fallen live a lot longer.
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Which begs the question of how come there is an unlimited supply of them. Creatures with long live spans have usually a very slow reproductive rate.