The Rules we use for cleaning out old forum messages:
If a message thread has less than 10 replies and if it was started longer than 12 months and the last reply was more than 4 months ago it is deleted.
If a message thread has less than 5 replies and if it was started longer than 8 months ago and the last reply was more than 4 months ago it is deleted.
The automatic cleaning action occurs every time the first page is loaded. There's no way we can except any messages and threads.
Therefore the only way to keep a message thread alive for long periods of time is to keep it active.