Zhè jiàn shì shuōláihuàchánɡ.
这件事说来话长。
It's a long story.
Notes:
这(Zhè): this
件(jiàn): measure word for thing
事(shì): thing
说来话长(shuōláihuàchánɡ): a long story to tell
Example:
A: 昨天发生了什么事?
Zuótiān fāshēnɡ le shénme shì?
What happened yesterday?
B:这件事说来话长。
Zhè jiàn shì shuōláihuàchánɡ.
It's a long story.