Time.time

Description

The time at the beginning of this frame (Read Only). This is the time in seconds since the start of the game.

Returns the same value if called multiple times in a single frame. When called from inside MonoBehaviour's FixedUpdate, returns fixedTime property. Try to avoid regular (frame) use of Time.time. It is more intended to supply the time the game has been running, and not time per frame.

Note: Time.time starts once all Awake functions have finished. The time value will be undefined during Awake functions.

using UnityEngine;
using System.Collections;

// Instantiates a projectile off every 0.5 seconds,
// if the Fire1 button (default is ctrl) is pressed.

public class ExampleScript : MonoBehaviour
{
    public GameObject projectile;
    public float fireRate = 0.5f;
    private float nextFire = 0.0f;

    void Update()
    {
        if (Input.GetButton("Fire1") && Time.time > nextFire)
        {
            nextFire = Time.time + fireRate;
            GameObject clone = Instantiate(projectile, transform.position, transform.rotation) as GameObject;
        }
    }
}