LifeSpan.cs

using UnityEngine;
using System.Collections;

public class LifeSpan : MonoBehaviour
{
    [SerializeField, Tooltip("How long this GameObject lives before dying, 0 = forever.")]
    private float initialLifeSpan = 0;

    private IEnumerator lifeSpanExpiredCoroutine = null;

    private void Start()
    {
        SetLifeSpan(initialLifeSpan);
    }

    /// <summary>
    /// Set the life span of this GameObject. When it expires, the GameObject will be destroyed.
    /// </summary>
    /// <param name="lifeSpan">The life span, in seconds, before this GameObject is destroyed. If 0, the timer is cleared and the GameObject will not be destroyed</param>
    public void SetLifeSpan(float lifeSpan)
    {
        if (lifeSpanExpiredCoroutine != null)
        {
            StopCoroutine(lifeSpanExpiredCoroutine);
        }

        if (lifeSpan > 0f)
        {
            lifeSpanExpiredCoroutine = LifeSpanExpired(lifeSpan);

            StartCoroutine(lifeSpanExpiredCoroutine);
        }
    }

    private IEnumerator LifeSpanExpired(float lifeSpan)
    {
        yield return new WaitForSeconds(lifeSpan);

        Destroy(gameObject);
    }
}