How long this takes varies greatly.
Now that I am working on my WIP (after putting aside the 50,000 words I wrote in late 2007 and early 2008 to write a back story novel first) I'm dealing with having to revise and/or replace much of the 50,000 words I have. During this process, which is slow going, yet productive and enlightening, I've been thinking about how much this story has changed, how much these characters have changed.
And I've come to realize I need to chart out where I go from here, once the 50,000 words are revised. So I decided yesterday while grabbing a quick bite for lunch, that when I got home from work I would write my outline for the novel.
Easy enough, right? Not really. I did manage to write an Outline last night but I only made it as far as outlining what happens in the current 50,000 words. It does, at least, contain some of the revisions I want to make. But I have to now continue forward and outline what I have yet to write at all.
The hard part is figuring out how to do this. Since I write from beginning to end, in order, do I outline this way? Or do I pick a POV character and outline that story line through to the end. And then pick the next POV character and do the same until each POV character's tale has been outlined?
Can I outline them individually like that? I think so, for a part of the remaining story. But for the final section of the book, where the actions of one greatly impact the reactions of another...I'm not so sure.
By the way, I'm doing a scene by scene outline so it will end up being quite a few pages. At some point, though this will be after I've finished the novel, I'll have to write a synopsis for an Agent. I don't even want to think about that now though. The Outline is by itself quite an interesting challenge.