I noticed that the field showed up after I had 100 posts. But I wasn't checking consistently, so it could have been much earlier. I'm also not sure if the Custom User Text is even linked to post counts. It could be linked to time as a member, for all I know.
So basically I'm saying "I have no idea" but at least you know that someone has read your post and has been through the same thing.
Thanks for the suggestion. I read another forum that is "powered by vBulliten" and the custom user text field is directly below the birthdate field. I'm not seeing it on my "edit profile" page here. Maybe you have to have a minimum number of posts now...