Usually, a thread that isn’t read gets ‘aged’ into being ‘already read’ after about two hours. At least during the day; in the early morning, the time is longer. There’s several differences apparent to the user: whether the thread title is shown in boldface or not in the list of threads; which article comes up when you hit “go to first new post”; and how long the thread shows up in the list of subscribed threads in the User CP.
Today, that time limit seems to have changed. I’m not sure what it’s changed to, since I haven’t seen any aged off the list. It’s now after 2 p.m. local and threads last posted to at 9:30 this morning are still in boldface. And threads from that time are still showing up in my User CP page. Perhaps they’ve changed it so only threads that have actually been read get that treatment. I know this is how most other message boards do it, but those don’t have the volume this one does.