I noticed that it depends on how much traffic the forum gets. If the forum has lots of activity, it gets indexed more quickly. It also seems to matter if you use the Google webmaster tools like the sitemap. I notice that soon after logging into the webmaster tools to check on *whatever*, Google crawls most of my sites.
I also see a direct correlation between how busy the forum is and how many spam registrations there are. More legitimate activity, more spammers register every day.