We are going to start "retiring" threads in the _chat topic once they are over 30 days old.
What this means is that once they are 30 days old (since last contribution rather than since starting) you won't be able to add messages to them any more - they'll still be in the archive search, they just won't have the "add a message" box at the bottom, and they won't be in the thread lists under the topic. This list is at over 8500 entries currently and becoming useless.
If archive searching starts to slow down, we might start deleting chat threads from the board altogehter after a certain time. As the content of these threads tends to be chats (surprisingly enough) rather than requests for information, there's not a huge amount of point keeping them for future reference.
On the subject of chat, we are currently test-driving chatroom software with a view to putting a live chatroom up on the site - for them as wants.