DestroyObjectOverTime.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class DestroyObjectOverTime : MonoBehaviour
{
    public float lifeTime; // How long the object should live
    // Start is called before the first frame update
    void Start()
    {

    }

    // Update is called once per frame
    void Update()
    {
        lifeTime -= Time.deltaTime;// Subtract Time.deltaTime from lifeTime

        if(lifeTime <= 0)// If lifeTime is less than or equal to 0
        {
            Destroy(gameObject);// Destroy the game object
        }
    }
}