How to make behaviour respond to a key press but not a hold?

I'm creating a game which relies on a weapon rotating in intervals around an object. The object will rotate when the specified key is pressed but if I hold it then the object will continue to rotate until the key is released. Does anyone know how I can stop it from rotating further than the interval by holding. Here is my code:

public class WeaponRotation : MonoBehaviour {

 // Use this for initialization
 void Start () {
 
 }
 
 // Update is called once per frame
 void Update () {
     if (Input.GetKey("left"))
         {
         transform.Rotate(0, 0, 45);
 }
     if (Input.GetKey("right"))
         {
         transform.Rotate(0, 0, -45);

} } }

If anyone could help it would be appreciated and any sample code would be a great help.