《史记·七十列传·大宛列传》
汉朝大致一年要派出的使者，多的时候十余批，少的时候五、六批。远的地方，使者八、九年才能回来，近的地方，几年就可以返回来。
汉率一岁中使多者十馀，少者五六辈，远者八九岁，近者数岁而反。
The Han Dynasty sent out envoys every year, sometimes as many as ten and as few as five or six. It took eight or nine years for envoys to return from distant places and a few years to return from nearer ones.