When is the best time to display ads in phone applications

Many online resources suggest you display ads when the app is launched, or when an important action is called (ex. image is being saved), but is it really efficient or rather off-putting?

The reason I am raising this question, is due to massive concurrence on Google Play and Apple App Store. Users have a lot of choice and if your ads are too annoying, but the functions you are offering are the same as someone else, why stay with you?

I would like to hear your thoughts on this.