Mouse Hold Down

So basically, when I hold my mouse button down, I want the timer to increase until it hits a certain time, however, it is only working if I click it, any ideas?

 var chopTime = 0.0;
 
 function Update () 
 {
     if (Input.GetMouseButtonDown(0)) 
     {
         //increment holdBreathTicker by deltaTime
         if (chopTime < 10)
         {
           chopTime += Time.deltaTime;
           }
     }
 }