When are the forums pruned?

The number of posts seems to be building at a pretty speedy rate, and I was wondering when the topics that are more than 5 months old would be pruned?

I've been using a DSL line for a year now and the forums seem to slow down for me lately within the last few months. Every other site is fine. Is it possible for some of the threads to be pruned anytime soon? Mainly the older threads that no one reads anymore.

Hopefully never again :D

I've not noticed this slowdown though. Pruning was done on previous servers to keep the database size down due to space limitations, but that's hardly necessary now on the current server.