Quick State Machine Question c#

I was following the BZA tutorial for spawning enemies and everything is working great but is there a way (or did I miss something along the way?) to make it cycle and spawn a new enemy after a certain amount of time after the enemy in that spawn point dies?

here is the script at the moment.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class ZombieSpawn : MonoBehaviour {
 
     public enum State
     {
         Idle,
         Initialize,
         Setup,
         SpawnMob
     }
 
     public GameObject[] zombiePrefab;
     public GameObject[] spawnPoints; // array holds a ref to all spawnpoints in the world
 
     public State state;    // local variable that holds our current state.
 
 
     void Awake()
     {
         state = ZombieSpawn.State.Initialize;
     }
     // Use this for initialization
     IEnumerator Start () {
         while(true)
         {
             switch(state)
             {
             case State.Initialize:
                 Initialize();
                 break;
             case State.Setup:
                 Setup();
                 break;
             case State.SpawnMob:
                 SpawnMob();
                 break;
             }
 
             yield return 0;
         }
     
     }
     
     private void Initialize()
     {
         Debug.Log ("Initialize Function");
 
         if(!CheckForMobPrefabs ())
         {
             return;
         }
         if(!CheckForSpawnPoints())
         {
             return;
         }
 
         state = ZombieSpawn.State.Setup;
     }
 
     private void Setup()
     {
         Debug.Log("Setup Function");
 
         state = ZombieSpawn.State.SpawnMob;
     }
 
     private void SpawnMob()
     {
         Debug.Log("Spawn Mob");
 
         GameObject[] gos = AvailableSpawnPoints();
 
         for(int cnt = 0; cnt < gos.Length; cnt++)
         {
             GameObject go = Instantiate(zombiePrefab[Random.Range(0, zombiePrefab.Length)],
                                         gos[cnt].transform.position,
                                         Quaternion.identity
                                         ) as GameObject;
             go.transform.parent = gos[cnt].transform;
         }
 
         state = ZombieSpawn.State.Idle;
     
     }
 
     //check to see if we have at least one mob prefab to spawn
     private bool CheckForMobPrefabs()
     {
         if(zombiePrefab.Length > 0)
             return true;
         else
             return false;
     }
 
     //check to see if we have at least on spawn point
     private bool CheckForSpawnPoints()
     {
         if(spawnPoints.Length > 0)
             return true;
         else
             return false;
     }
 
     //generate a list of available spawnpoints that do not have any mobs childed to it
     private GameObject[] AvailableSpawnPoints()
     {
         List<GameObject> gos = new List<GameObject>();
 
         for(int cnt = 0; cnt < spawnPoints.Length; cnt++)
         {
             if(spawnPoints[cnt].transform.childCount == 0)
             {
                 Debug.Log("Spawn Point Available");
                 gos.Add(spawnPoints[cnt]);
             }
         }
 
         return gos.ToArray();
     }
 }