guys i need some help

i have tried over an over to post this question on the answers.unity3d website but i could post it for some reason tried many time .
please help

    using UnityEngine;
         using System.Collections;
       
         public class Generate : MonoBehaviour {
       
             public GameObject lightnest;
             public float initialDelay;
             public float finalDelay;
             public float rampDuration;
       
             protected float _delay;
             protected float _runTime;
             protected float _timeSinceSpawn;
             // Use this for initialization
             void Start () {
                 _delay = initialDelay;
                 _runTime = _timeSinceSpawn = 0.0f;
              
             }
       
             // Update is called once per frame
             void Update() {
               
                 _runTime += Time.deltaTime;
                 _timeSinceSpawn += Time.deltaTime;
                 _delay = Mathf.Lerp(initialDelay, finalDelay, _timeSinceSpawn / rampDuration);
                 if (_timeSinceSpawn > _delay) Spawn();
              
             }
             protected void Spawn()
             {
                 _timeSinceSpawn = 0.0f;
                 Instantiate(lightnest);
             }
          
       
         }

So this is the code which i use to generate the pipe thingys ,but after it genrates it i want to destroy it after its out of screen or any other possible way ,maybe after 3sec or any way possible and i want this pipe thingys to generate randomely in different sizes you get me ryt just like flappy bird

Take a look at the flappy bird example we provide on the asset store (its free!).
I think its under standard assets.

couldnt find it

I think its this one Unity Asset Store - The Best Assets for Game Making

thank you