Google Play has updated its developer program policies with a number of new rules that will be phased in over the coming months, including some new restrictions on how apps use ads.
Beginning September 30, unexpected full-screen interstitial ads that pop up when users have chosen to do something else will not be allowed.
On top of that, users must be able to quit out of full-screen interstitial ads within 15 seconds, unless they specifically opted into it (for an in-game reward, for example) or it doesn't interrupt normal use of the app.
Google gave an example of an acceptable non-skippable ad being one that appears after the score screen following a gameplay session.
Additionally, if an app targets children, users must be able to skip advertising the interferes with normal app use within five seconds.