Slider that reduces value based on timer

I have created a slider that reduces based on the timer of my power up(slowing time for enemies). What I can't seem to be able to do is make the timer go down only when the power up is Active. This is the code for the Enumeration

  private IEnumerator slowDownTimer(float duration, Slider slider)
 {
     // Slows time for enemies
     GameManager.enemyTime = 0.1f;
     Debug.Log("space key was pressed");
 
     //Timer of three seconds, and reset of enemy speed
     yield return new WaitForSeconds(duration);
     GameManager.enemyTime = 1f;
 
     // Decreases the Slider value over time
     if (slider != null)
     {
 
 
         float timeScale = (slider.value / (duration / 0.5f));
         while (slider.value >= 0)
         {
             slider.value -= timeScale;
             yield return new WaitForSeconds(duration);
             if (slider.value <= 0)
             {
                 break;
             }
         }
         yield return null;
     }
 
 }