Okay, I found a chart listing the years per turn for the game! This should help us do the math, Siredgar.
PERIOD YRS/TRN TURNS
4000 BC - 2750 BC 50 25
2750 BC - 1750 BC 40 25
1750 BC - 750 BC 25 40
750 BC - 250 AD 20 50
250 AD - 1250 AD 10 100
1250 AD - 1750 AD 5 100
1750 AD - 1950 AD 2 100
1950 AD - 2050 AD 1 100
540 turns total. YRS/TRNS don't vary with difficulty level.
basically we have a period of 6050 years, and a total of 540 turns with years per turn gradually decreasing from 50 to 1.