How to set a firerate on this?

try -

 using UnityEngine;
 using System.Collections;
 
 public class test : MonoBehaviour
 {
     public float fireRate;
     
     private float nextFire; //set the time between shots
 
     void Update()
     {
         if (Input.GetButton("Fire1") && Time.time > nextFire) 
         {
             nextFire = Time.time + fireRate;
             GetComponent.<AudioSource>().pitch = 2;
             GetComponent.<AudioSource>().Play();
             var ray: Ray = Camera.main.ViewportPointToRay(Vector3(0.5,0.5,0)); var hit: RaycastHit; var dir: Vector3; if (Physics.Raycast(ray, hit)){ // if something aimed // find the direction from spawnPoint to the target 
                 dir = (hit.point - spawnPoint.position).normalized; 
             } 
             else { // if there's nothing, use the ray direction 
                 dir = ray.direction; // spawnPoint.forward works too (see note below) 
             }
             var rot = Quaternion.FromToRotation(rocketPrefab.forward, dir); // instantiate the rocket 
             var rocket = Instantiate(rocketPrefab, spawnPoint.position, rot); // launch the rocket 
             rocket.GetComponent(Rigidbody).velocity = dir*rocketSpeed; 
         }
 
 
     }
 }
 

:)

Try this out...

 public float delay = 0;
 public float rate = 1f;
 
 void Update()
 {
    if (Input.GetButton("Fire1")) 
    {
        InvokeRepeating("Fire", delay, rate);
    }
 }  
 
 void Fire()
 {
    //Instantiate prefab or wateva
 } 

Hope it helps :)