Global time triggered script

How can i add a script to my project wich is executet in every scene the whole time the game is active.

As an example I want to pop up some ads after every x minutes, don’t matter wich level is currently played.

One previus solution for me was to add a script to every scene, but there must be a easier way to do this, because Ihave now about over 50 scenes.

can I add an empty gameobject to my first scene (splash screen) with the script on it, and tell it somehow to stay alive the whole game?

Thx

Thx to savlon, that was what I was searching for.

For other people who are searching for the same solution, I post some of my code here:

public class AdManager : MonoBehaviour {


	public float TimeToShowAd = 600f;   // in Seconds

	private float TimeSinceStart = 0;

	private static AdManager instance = null;
	// Handling to stay alive forever
	void Awake(){

		Advertisement.Initialize (gameID, true);

		if(instance == null){ 	// initialize only  one instance.
			instance = this;
			GameObject.DontDestroyOnLoad(this.gameObject);
		} else {				// Destroy unused instances.
			GameObject.Destroy(this.gameObject);
		}
	}

void Update () {

		
			TimeSinceStart = TimeSinceStart + Time.deltaTime;
			if (TimeSinceStart > TimeToShowAd) {
				TimeSinceStart = 0;
				ShowAd();
			}


		}
	
}