The Years of Dusk should have lasted ten years based on the codex, after which, I presume the Age of Dawn started, but as to how long the Year of Steel (the previous era) lasted, I don't believe it is specified.

I made a terrible diagram to try and understand things myself:


Age of Myth

    |
    v

Age of Steel

    |
    v

Years of Dusk (10 years)
	[i]Scourge of Nemedia 
	Fall of Altantis, Iiram and Thule (end of this period)[/i]

    |
    v 

Age of Dawn
	[i]56th year = present[/i]