I show an interstitial at these points:
Starting a new game
Quitting the game to return to the main menu
Every three levels
At GameOver (if rewarded ad not selected)
When the GameOver screen shows, I give the player a choice to watch a rewarded ad for an extra life to continue playing for a high score.
wow, that sounds like a lot of ads.
i am also not sure that on Gamestart you may show ads but even if you are allowed, this is a very bad idea in my opinion.
for games, i would show only an Ad when the game is finished. that's enough in my opinion so the player will enjoy the game.
i would show banners only in menus where you can purchase items or main screen but not inside the game.
I also offer an IAP to remove ads for a small fee.
the question is this, how many IAP do you have per month?
in my opinion, selling the No Ads in-app purchase for a few $ is a very bad business idea and the reason is this, the only user that will purchase a no ad package in a game is your best client because he really likes the game and he plays it a lot but the ads are annoying so he wants to remove them. so you charge him for 1$ and your best client only made you 0.85$ (0.15 goes to google). now imagine if you would show him much much less ads like 2-3 ads per whole game interaction (from loading your app to closing it) you can make much more than 1$ in a long period of time instead you give your best client the opportunity to get rid of all ads for 1$ and we all know that ads is the big money compared to IAP.
so bottom line, don't offer IAP that remove ads (you can offer to buy stuff like unlock levels or new cars, depending on your game) instead show fewer ads and in the end you will earn much more from your game.