I'd imagine that waitlists are filled right away, but that doesn't mean it's easy to fill them. Since a waitlist is an all-or nothing proposition, the only way you can actually get a waitlist filled is if someone cancels a reservation that includes all of your dates, and you're first in line. When you think about it, that can be tricky, especially if you're talking more than maybe two nights.
In my case, I saw a cancellation pop up for XMAS at VGC and grabbed it without a waitlist. It was Sun-Thu (4 nights). Seems like there must be a lot of waitlists at Christmas, right? But, unless those waitlists were for exactly Sun-Thu, or 3 or 2 or 1 night in between, then the waitlists wouldn't be filled. Anyone who wanted Saturday was not filled. Anyone who wanted Thursday night was not filled.
This "all or nothing" issue with waitlists is I think what causes a lot of frustration. That, and the 2-waitlist limit. When I was looking, I was flexible with dates, but needed to use points by Mar 1, so I needed to constantly scan availability over the next few months. I ended up automating it to check every 6 hours. When I posted about it on another forum, they all encouraged me to keep working on it and I made it into a website.