Check GameInfo\CIV4GameSpeedInfo.xml.
The GameTurnInfos section specifies how many turns make up a game, divided into sections that also specify how many months each turn represents. This is how you get 75 turns at 480 months (40 years) per turn, then 60 turns at 300 months (25 years) per turn, and so on. The total number of turns in a game is the sum of all the iTurnsPerIncrement values. A regaular BtS game on GAMESPEED_NORMAL lasts for 75 + 60 + 25 + 50 + 60 + 50 + 120 + 60 = 500 turns.