Problema com Banner Interstitial

Como fazer banner interstitial Abrir a cada 5 minutos.

Código que estou usando.

AdRequest adRequest = new AdRequest.Builder().build();

    // Prepare the Interstitial Ad
    interstitial = new InterstitialAd(MainActivity.this);

// Insert the Ad Unit ID
interstitial.setAdUnitId(getString(R.string.admob_interstitial_id));

    interstitial.loadAd(adRequest);

// Prepare an Interstitial Ad Listener
interstitial.setAdListener(new AdListener() {
public void onAdLoaded() {
// Call displayInterstitial() function
displayInterstitial();
}
});

public void displayInterstitial() {
// If Ads are loaded, show Interstitial else show nothing.
if (interstitial.isLoaded()) {

        interstitial.show();

    }
}