It's so long since ...：好久...的意思。’s还原为It is so long since，它另外一个常见的结构是It has been so long since。

【例句】
It's so long since it rained that the river-bed is dry.
很久没有下雨了，河床都已乾涸。

For it's been so long since I have seen you.
已经很久没有见过你，我甚至已经忆不起你的模样。