exhaust manifold ?
 

What wpuld cause an exhaust manifold to crack and how fast do I need to change it

It just happens. The heating cooling cycles cause them to expand and contract and they just crack. You can replace it with another stock one that may last, or you can get an aftermarket one that won't do that.

i found out the hard way that washing out your engine bay at the car wash will do it to[dummy]

stock manifold?

Ya sorry its a stock manifold

There's a guy on CompD who has a pic up of an ATS manifold with 4 BIG cracks in it, and there are quite a few people who's crack.

But to answer the poster's question, either weld it up or get a new one.