Changing input.getkeydown to change every few seconds?

Hello I really could use some help with this!

Ii have a script (which I will link below) That cross fades two cameras together but only if I press the space bar.

How will I be able to change it so instead of pressing space bar it changes every couple of seconds?

Here is the script:

using UnityEngine;
using System.Collections;

public class CrossFadeExample : MonoBehaviour
{
		public Camera camera1;
		public Camera camera2;
		float fadeTime = 2.0f;
		public AnimationCurve curve;
		private bool inProgress = false;
		private bool swap = false;

		void Update ()
		{
				if (Input.GetKeyDown ("space")) {
						DoFade ();
				}
		}

		void DoFade ()
		{
				if (inProgress) {
						return;
				}
		
				inProgress = true;
		
				swap = !swap;
				StartCoroutine (delayLine ());
		
	

	
		}

		IEnumerator delayLine ()
		{
				Camera tmpCam1;
				Camera tmpCam2;
		
				if (swap) {
						tmpCam1 = camera1;
						tmpCam2 = camera2;
				} else {
						tmpCam1 = camera2;
						tmpCam2 = camera1;
				}
				print ("going to do something:");
				StartCoroutine (ScreenWipe.use.CrossFade (tmpCam1, tmpCam2, fadeTime, curve));


				yield return new WaitForSeconds (2.1f);
				inProgress = false;
				print ("done");
		}
}

Any help is appreciated!

Try using InvokeRepeating

More specifically, replace your Update function with this:

void Start()
{
      InvokeRepeating("DoFade", 2f, 2f);
}