In order to maximize the number of unique users that see ads each day consider the typical user flow and try to insert ads where a majority of users will see them. Keep in mind that if you show your first ad at minute 5 of a user session, you'll miss out on monetizing all the users that only spend 1-4 mins in the app. By this logic, try to insert your first ad as soon as possible where it fits within the flow of the app. This is probably the most important step to maximizing interstitial ad revenue.
The best times to show interstitial ads are at natural pauses in the app. Pauses may come between levels, after a “failed” game, or once the user reaches a certain "high" in a continuous game. These pauses or breaks in game play are generally the occasions when a user is most receptive to an ad, meaning better ad performance and higher eCPMs. Avoid showing ads at times when a user is very actively engaged in your app.