Tower defense auto fire

So right now i can spawn a tower that will shoot an enemy only once, once it enters it’s collider. but i want it to keep shooting the enemy with a timer reload speed that i can set. how would i go about making this? here is what i have right now:

using UnityEngine;
using System.Collections;

public class Tower : MonoBehaviour {
// The Bullet
public GameObject bulletPrefab;

private Transform target;

void OnTriggerEnter(Collider co) {

	if (co.GetComponent<Monster> ()) {

		GameObject g = (GameObject)Instantiate (bulletPrefab, transform.position, Quaternion.identity);
		g.GetComponent<Bullet> ().target = co.transform;

	}

}

}

There is a function OnTriggerStay() with the same implementation, it is called every frame the collider stays within the trigger.

You can either use an IEnumerator or a countdown to fire every second or two.