public function duration()
    {
        $duration = 0;
        foreach ($this->_tasks as $task) {
            $duration += $task->duration();
        }
        return $duration;
    }
public function duration()
    {
        $duration = 0;
        foreach ($this->_tasks as $task) {
            $duration += $task->duration();
        }
        return $duration;
    }
Returns the total duration.

@return integer The total duration (in microseconds).