This would depend on how you're rotating the object in the first place. If you're using physics and torque, a configurable joint would fit well.

If you're rotating by manipulating the transform directly, without physics, it's probably best to maintain a variable containing the angle, and limit it yourself, perhaps using the Mathf.Clamp function. For example:

var angle = 45.0;

function Update() {

    angle += Input.GetAxis("Vertical") * Time.deltaTime * 10;
    angle = Mathf.Clamp( angle, 10, 80 );

    transform.localRotation = Quaternion.AngleAxis(angle, Vector3.right);
}

If you need a more detailed answer, please edit your question to include more detail about how you're rotating the object.

Have a look at the MouseLook script in Standard Assets, which has a rotation clamp function that would work perfectly in this situation as long as you're directly setting the rotation and not using physics. If you need a Javascript version of MouseLook.cs, it's here.