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.

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

var projectile : GameObject;
var fireRate = 0.5;
private var nextFire = 0.0;

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

using UnityEngine;
using System.Collections;

public class ExampleClass : 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;
        }
    }
}

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