Game retention and correct ad placement

Does anyone have any feedback or study on the best way to implement ads in your game? Would a timer be best like play and ad at every natural break point every 2 minutes, 5 minutes, 10 minutes of gameplay? Maybe consistent predictable ads so a player knows when they are coming.

What frequency is best? Whats too many before a user gets annoyed?

Thoughts?
Thanks,

 

Figure out how long your average gameplay session usually lasts (is it 30 seconds, 2 minutes, 5 minutes?) and work your way from there. Natural break points are a good start, such as in between levels, but if your game doesn't have levels and breaks in between, be wary of implementing forced interstitials that interrupt gameplay.

Best case scenario you should be using rewarded ads instead of (or along with) interstitials.

Through quick googling I found these articles:
https://appsamurai.com/best-practices-of-video-ad-placement-in-mobile-games/
https://gameanalytics.com/blog/popular-mobile-game-ad-formats.html

Especially the gameanalytics one has some great info.

EDIT: Unity also has articles about this:
https://unityads.unity3d.com/help/resources/best-practices