how to make an object spin continuously?

In the Update function I use: (C#)

 transform.Rotate (new Vector3 (0, 0, 100) * Time.deltaTime * 20);

The speed and direction can be changed by changing the numbers. I'm using this for a propeller/fan blade rotation so I think faster works better for now.

Add a script to it that applies a continuous rotation on one of its axes in the Update() method.