First 25 turns = 50 years (4000 BC)
Next 25 turns = 40 years (2750 BC)
Next 40 turns = 25 years (1750 BC)
Next 50 turns = 20 years (750 BC)
Next 100 turns = 10 years (250 AD)
Next 100 turns = 5 years (1250 AD)
Next 100 turns = 2 years (1750 AD)
All turns yearly after that (1950 AD)
So, adding it all would be 540 turns
-MZ
P.S. I liked the Civ2 breakdown better and in fact use it in my personal mod which I occasionaly play with