Help ! Rotate C# script !

*** Sorry for my English ! I from Polish . ***
OK.
I have something like this:

public float rotationSpeed = 100;

void Update ()
{
	if (Input.GetKeyDown (KeyCode.A)) 
	{
		transform.Rotate(new Vector3(0,rotationSpeed * Time.deltaTime,0));
	}

(it’s a fragment)
What i must doing if rotate continued all time ?
Now my object rotates only one time when i click “A” .
TY !

Reading the docs makes sense: