Admob: Interstitial ads showing every time

i'm finishing a video application and i'm displaying interstitial ads when leaving the video activity. I just want to show it once every X minutes, but it seems to be showing every time i leave that screen.

This is my activity code.

OnCreate:

  @Override
  public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  Bundle b = getIntent().getExtras();
  videoId = b.getString("videoId");
  setContentView(R.layout.youtube_player);

  interstitialAd= new InterstitialAd(this);
  interstitialAd.setAdUnitId(getString(R.string.interstitial_full_screen));
  AdRequest adRequest = new AdRequest.Builder().build();
  interstitialAd.loadAd(adRequest);
  etc...

OnBackPressed:

@Override
public void onBackPressed() {
    ShowAds();

}

private void ShowAds() {
    if (interstitialAd.isLoaded()) {
        interstitialAd.show();
        interstitialAd.setAdListener(new AdListener() {
            @Override
            public void onAdClosed() {
                super.onAdClosed();
                finish();
            }
        });
    }else{
        super.onBackPressed();
    }
}`

of course, it is setted like this into AdMob:

Note: my application is not published, so it is showing the "preview" o "sample". I am using my ad unit ID:

thanks,

Two options :

Interstitial ads show when I am leaving current Activity but only If I completed X minute on current Activity.

boolean isAdShow=false;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    int minute=1;  // X minute

    isAdShow=false;
    new CountDownTimer(minute*60000,1000) {
      @Override
      public void onTick(long millisUntilFinished) {

      }

      @Override
      public void onFinish() {
          isAdShow=true;
      }
  }.start();
}

private void ShowAds() {

   if (interstitialAd.isLoaded() && isAdShow) {
       interstitialAd.show();
       interstitialAd.setAdListener(new AdListener() {
          @Override
          public void onAdClosed() {
              super.onAdClosed();
              finish();
          }
       });
    }else{
       super.onBackPressed();
    }
}

Don't wait for user to press back button and leave current Activity, just call ShowAds() from onFinish() method of Timer.

I'll recommend to use 1^st one because it not violate AdMob Ad policy and user experience too.

EDIT

You can also use X times counter, like when X = 3 i.e After 3 times call of onCreate() method make eligible to show Ad.

public static int adCounter;
public static final int TIME_COUNTER=3;  //Should be always greater than zero

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    adCounter++;
}

private void ShowAds() {

  if (interstitialAd.isLoaded() && adCounter%TIME_COUNTER==0) {
    interstitialAd.show();
    interstitialAd.setAdListener(new AdListener() {
       @Override
       public void onAdClosed() {
           super.onAdClosed();
           finish();
       }
    });
   }else{
    super.onBackPressed();
  }
}