Instantiate prefabs before it comes into view

Im making a platform type game and I was trying to figure out how to Instantiate a prefab as the the player(object) about every 5 units in the y direction.

Ive got the actual Instantiate working

//////////////////////////////////////////// var prefab : Transform; var playerCam : Transform; var player : Transform;

function Update () { var playerCamY = playerCam.position.y; var abovePlayer = playerCamY+7; var belowPlayer = playerCamY-7;

var randPlaceX = Random.Range(-2,2); var randPlaceY = Random.Range(abovePlayer,abovePlayer+5);

Instantiate (prefab, Vector3(randPlaceX,randPlaceY,-0.1738763),Quaternion.identity); }

////////////////////////////////////////////////

But I just need a way of telling WHEN to Instantiate. Is there something like InvokeRepeating but for my own variable rather than seconds.

I Ultimately want to get the prefabs to Instantiate before it enters the frame then destroy it soon after it leaves the frame.

Any help would be greatly appreciated, Ive been scratching my head for ages on this one.....