Hi there.
I agree with drsnidely. Usually you should play the game until it becomes evident that one side is going to lose.
I play regularly with a group of equally-matched friends strategically and the game can still be undecided by turn 10 at times. Usually though, by that time one side will have a clear advantage and it would just be pointless to continue for a couple of hours for the unavoidable to happen.
I expect you should start to see how the game will go by turn 5, so plan for at least 7 turns and be prepared to fight it out for longer if strategy and chance dictate a close game.
Enjoy !