How about reading the documentation, that's what everyone, who answers question, does when they first start using a new core.
From the Product Guide for the FFT core.
This means you'll have to run the core generator for this with whatever settings you want to see what the latency is for your application (which we have no idea what that application is so how are we supposed to tell you the latency?)
Okay I'll take a guess...
15 clocks
If my guess is closest do I get a prize?