Delay Advertisements when switching between scenes?

So I have have a script that plays an add every 90 seconds. Every time you change scene (which is quite regular) An ad pops up.

Does anyone know how to delay this

Script:

 using UnityEngine;
 using System.Collections;
 using UnityEngine.Advertisements;
 
 public class ShowAds : MonoBehaviour
 {
     float timer;
     void Start()
     {
         Advertisement.Initialize ("<YOUR GAME ID HERE>"); //REMEMBER TO EDIT!!
     }
 
     void Update()
     {
         timer -= Time.deltaTime;
         ShowAd ();
     }
 
     public void ShowAd()
     {
         if (Advertisement.IsReady () && timer <= 0) 
         {
             Advertisement.Show ();
             timer = 90.0f;
         }
     }
 }