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.