rotate slowly!

To change the speed you need to multiply the vector you want to rotate around by a speed variable. Check if the y rotation is greater than or equal to the target angle. If it's not carry on rotating, if it is then stop rotating.

 public float speed = 10;
 public float angle = 30;
 private bool rotating = false;
 
 void Update ()
 {
     if(Input.GetKeyDown(KeyCode.Space) && !rotating) rotating = true;
     
     if(rotating && transform.eulerAngles.y <= angle)
     {
         transform.Rotate(Vector3.up * (speed * Time.deltaTime));
     }
     else if (rotating) rotating = false;
 }