code: public function getEstimate($key, $limit, $milliseconds)
      {
          $key = $this->getStorageKey($key, $limit, $milliseconds); 
          $newRatio = $this->getNewRatio($key, $limit, $milliseconds);
          $wait     = 0;
          if ($newRatio > $milliseconds) {
              $wait = ceil($newRatio - $milliseconds);
          }
          return $wait;
      }
docs: Get Estimate (doesn't require lock)
  
  How long would I have to wait to make a request?
  
  @param string $key  - A unique key for what we're throttling
  @param int $limit   - How many are allowed
  @param int $milliseconds - In this many milliseconds
  @return int - the number of milliseconds before this request should be allowed
  to pass