C# How do I get my turret to fire at player every few seconds while they are in range?

Basically I have a turret and a set range. Whenever the player enters that range I want my turret to fire at them once every few seconds an continue this as long as the player is in range. I have seen several examples how to do this, even in C#, but I have had little success so far. I tried using a Coroutine and IEnumerator, which worked as long as the player started out in range, but once they left range and came back it would do nothing. I tried InvokeRepeating, but it didn’t seem to delay the shots no matter what I set the time between calls to. The strip of code below seems to be the cleanest and it will shoot at the player while they are in range, but I need to add some sort of time delay to it.

using UnityEngine;
using System.Collections;

public class S_skullTower : MonoBehaviour {

	public Transform target;
	public float turretSpeed;
	public float fireRate;
	public float fireBallHeight;
	public GameObject fireBall;
	public float range;
	float distance;

	// Use this for initialization
	void Start () 
	{
		
	}
	
	// Update is called once per frame
	void Update () 
	{
		//Rotate turret to look at player.
		Vector3 relativePos = target.position - transform.position;
        Quaternion rotation = Quaternion.LookRotation(relativePos); 
       	rotation.x=0;
       	rotation.z=0;
        transform.rotation = Quaternion.Slerp(transform.rotation, rotation, Time.deltaTime * turretSpeed);
		
		//Fire at player when in range.
		distance = Vector3.Distance(transform.position,target.position);
		
		if(distance < range)
		{
			launchFireBall();
		}	
	}
	
	void launchFireBall()
    {
		Vector3 position = new Vector3(transform.position.x, transform.position.y + fireBallHeight, transform.position.z);
		Instantiate(fireBall, position, transform.rotation);
	}
}

Any direction or suggestions would be much appreciated.

You can add a new variable:

private float _lastShotTime = float.MinValue;

and then use:

if (distance < range && Time.time > _lastShotTime + (3.0f / fireRate))
{
    _lastShotTime = Time.time;
    //print(Time.time);
    launchFireBall();
}

This way, your tower shot speed will depend on fireRate. You can of course change 3.0f to any value.

Personally, I like to use some kind of timer with a float and Time.deltaTime. Something like:

if (distance < range)
{
    fireTimer += Time.deltaTime;
    if (fireTimer >= fireRate)
    {
        fireTimer -= fireRate;
        launchFireBall();
    }
}

Also, just a couple notes for you: instead of using Vector3.Distance (which uses a sqrt function internally) you could do something like:

sqrDistance = (transform.position - target.position).sqrMagnitude;

And then instead of checking against range, check against range², i.e.

if (sqrDistance < range * range)
{
    // Do something
}

Hope this helps!

EDIT: just a side note: as it stands, this code will shoot a fireball every X seconds that the player is within range. Once they are out of range that timer value will still be non-zero. You will want to set it back to zero when the player is out of range if you want the timer to be reset.