My answer probably isn’t what you’re hoping for and I don’t mean for it to be taken offensively…
I would definitely say it depends on what type of app you have. Try to figure out what your target audience is(population size and characteristics) and look for the similar niche apps and compare download numbers. Does your app mostly appeal to 80+ year old women? Is your app a calculator or some flashy match 3 game with social networking? Just think about how many(few) 80+ year old women are using smart phones or tablets.
Note: I am by no means saying your app is inferior, a calculator or appealing to only 80+ year old women. I simply used examples I thought would be obvious for making a point.