How to use the wagtailvideos.ffmpeg.get_duration function in wagtailvideos

def video_saved(sender, instance, **kwargs):
    if not ffmpeg.installed():
        return

    if hasattr(instance, '_from_signal'):
        return

    has_changed = instance._initial_file is not instance.file
    filled_out = instance.thumbnail is not None and instance.duration is not None
    if has_changed or not filled_out:
        with get_local_file(instance.file) as file_path:
            if has_changed or instance.thumbnail is None:
                instance.thumbnail = ffmpeg.get_thumbnail(file_path)

            if has_changed or instance.duration is None:
                instance.duration = ffmpeg.get_duration(file_path)

    instance.file_size = instance.file.size
    instance._from_signal = True
    instance.save()
    del instance._from_signal