As little as possible. No, really! I tend to sketch out the basics when I’m planning a new series, but the rest grows more organically as I write. I love being able to make stuff up as I go and add it to my “world notes” for future reference/consistency. It’s part of what keeps the world, and the writing process, fresh for me.
I’ve also been known to remark that one of the reasons I enjoy writing urban fantasy rather than, say, epic fantasy is that I get to be a little lazy about world building. I don’t have to make up the whole world, I just have to decide in what ways my world is different from the real world. I can use existing cities, countries, cultures, etc., and it gives me the opportunity to do something I really enjoy: research and learn new things.
The flip side is that, if I’m not careful, I can fall way down a deep, dark research rabbit hole and forget I’m supposed to be writing!