Rotating an object

This script will slowly increment a float to 5 when the key is pressed and will decrement back to 0 when released. Assign the float to your rotation speed and tweak the variables to suit your need.

Hope it's helpful!

     float delay = 0.3f; //Delay between each increment of speed
     float speedIncrement = 0.3f; //Amount of speed to increment
     float maxSpeed = 5f; //Maximum rotation speed
     
     float rotationSpeed;
     bool buttonPressed;
     
     IEnumerator IncreaseSpeed(float delay)
     {
         while (buttonPressed) 
         {
             while (rotationSpeed < maxSpeed)
             {
                 yield return new WaitForSeconds (delay);
                 rotationSpeed += speedIncrement;
             }
             rotationSpeed = maxSpeed;
             yield return null;
         }
     }
     
     IEnumerator DecreaseSpeed(float delay)
     {
         while (rotationSpeed > 0) 
         {
             yield return new WaitForSeconds (delay);
             rotationSpeed -= speedIncrement;
         }
         rotationSpeed = 0;
         yield return null;
     }
 
     void Update () 
     {
         if (Input.GetKeyDown(KeyCode.K)) //Change to your assigned button
         {
             buttonPressed = true;
             StartCoroutine(IncreaseSpeed(delay));
         }
         if(Input.GetKeyUp(KeyCode.K)) //Change to your assigned button
         {
             buttonPressed = false;
             StartCoroutine(DecreaseSpeed(delay));
         }
         Debug.Log(rotationSpeed);
     }