Round 0 (After settling).
Round 1, 4000 BC - 2920 BC (27 turns).
Round 2, 2920 BC - 875 BC (53 turns).
Round 3, 875 BC - 25 BC (34 turns).
Round 4, 25 BC - 600 AD (26 turns).
Round 5, 600 AD - 1100 AD (30 turns).
Round 6, 1100 AD - 1500 AD (40 turns, the end).
I have a little spare time now so...