It really totally depends on the gameplay. In my ball-track-builder one round might take a minute - the player places items to guide the ball and then let the ball go. Then it depends on the player how long he or she wants to play hoping to gain points.
Several games I have take more or less about half an hour. Those are games the player might want play several times because there is the incentive of getting better with experience. It's not always easy to measure. Building Bill Burton Road has a clear ending when the roads reaches the destination whereas a sandbox game like Building Baroque just takes as long as the player likes building.
Narrative games would typically be longer as they will not be played them more than once. My detective game, which is my only narrative game takes about 2 hours for all cases (But I'm planning to make more cases - feel free to suggest a murder :-) ).