Timer between ads displayed

Im using google admob and to make my users happy and not fed up with ads i want to space them out. I have a working code which i know works as in the console it says a message whenever one is supposed to display. Here is my code and i want to see how to add a 240 second timer between ads shown. Please help :).

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using GoogleMobileAds.Api;
 
 public class AdmobManager : MonoBehaviour
 {
     private BannerView bannerView;
 
     [SerializeField] private string appID = "";
     [SerializeField] private string bannerID = "";
     [SerializeField] private string regularAD = "";
 
     private void Awake()
     {
         MobileAds.Initialize(appID);
     }
 
     public void OnclickShowBanner()
     {
         this.RequestBanner();
     }
 
     public void OnClickShowAd()
     {
         this.RequestRegularAd();
     }
 
     private void RequestRegularAd()
     {
         InterstitialAd AD = new InterstitialAd(regularAD);
 
         AdRequest request = new AdRequest.Builder().Build();
 
         AD.LoadAd(request);
     }
 
     private void RequestBanner()
     {
         bannerView = new BannerView(bannerID, AdSize.Banner, AdPosition.Bottom);
 
         AdRequest request = new AdRequest.Builder().Build();
 
         bannerView.LoadAd(request);
     }
 }