last night the chat thread got closed because of arguing, in that case people are not permitted to start up a new one.
Normally generally speaking they get closed when a mod thinks they're big enough could be anywhere between 3000-5000 posts really. When it's decided to close the thread, those posting in the Chat thread usually decide amongst themselves who will start the new one and it's started pretty much as soon as the old one is closed
The reason they get closed when they get too big is that bigger threads can slow the forum a bit I think