What Yakobu said is correct. The number of turns is dependant on how long it takes to fulfill the campaign goals, ie hold a certain amount of provinces and eliminate one or two of the neighbouring factions. Ie everything from 1 to 200+ .

When you complete the goals you are asked after the victory video if you want to continue playing. The campaign then becomes a standard long campaign, ie hold 45 provinces and Jerusalem or some other province.