The game turn is probably stored as an unsigned 32 bit integer, so the theoretical upper limit would be 4,294,967,295 turns based solely on that.
But you'd die of old age long before then.
The answer for any specific mod is that the turns until the time limit depends on how the mod has that set. It is controlled in CIV4GameSpeedInfo.xml in the GameTurnInfos section for each game speed. The total turns before the end of the game for the time victory is the sum of the iTurnsPerIncrement values for all of the GameTurnInfo parts.