It´s really impossible to tell without knowing whats the app about. An ebook reader could have an average of 30 mins, while a flashlight only a couple seconds.
We got games with average sessions ranging from 1 to 18 mins, it really depends on the type of game…
I would think utilities apps are at a great disadvantage. Reason being most utility apps consists of setup and search screen. For setup, they will take a very short time to set them up and then exit the app. Phones with slow internet connection will get delayed interstitial ad popping up and viola you get rejected from gp. It seems interstitial ad format is not a good fit for utility apps.