//variable
private InterstitialAd interstitialAd;

//code implementation
//in millisecond, for 2 minutes = 120000 millisecond
        new Handler().postDelayed(new Runnable() {
            @Override
            public void run() {

                interstitialAd = new InterstitialAd(MainActivity.this);
                interstitialAd.setAdUnitId(getString(R.string.admob_interstitial_id));
                AdRequest adRequest = new AdRequest.Builder().build();
                interstitialAd.loadAd(adRequest);
                interstitialAd.setAdListener(new AdListener() {
                    public void onAdLoaded() {
                        if (interstitialAd.isLoaded()) {
                            interstitialAd.show();
                        }
                    }
                });

            }
        }, 120000);