It's common thing for exploration and mystery games to have long play time. The time it requires for player to explore and unravel mystery is usually longer than action games. IOW those games are slowly paced.

Unfortunately, the same couldn't be said about horror. Horror couldn't last long cause player would adapt to the atmosphere after some time.

Oh about your main question : my games are usually action games therefore they last for 1 hour at max. If it requires longer than that, I'd implement save system.

Like DarkBloodbane, my games are usually action based and the play session range from 5 to 30 minutes. Red Balloon of Happiness is a horror game you can complete in 15 minutes.

Kinda depends. I've made game jam games you can complete in like 10. Gravity Control takes maybe an hour? Give or take I never really speedran it.

My newest game I'm putting more time in I intend to take maybe 6-7 hours in total but it's years from completion.

The length of the game doesn't really matter though so much as the content of the game itself. You also have to account for people that just skip things, those that go into everything, etc....

 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 :-) ).

I think that's very relative. It will only drive away new players if your game is unfair. But I don't think that's the case for you, I believe. Normally my games last a maximum of forty minutes to an hour and a half. But that's because I made my games quite difficult. Even so, I think your game is within a pretty normal time frame, to be honest. I don't think there should be a time limit for a game. Minecraft has an average gameplay time of two or three hours, and Minecraft is a huge game. I wish you much success.