I work on about a year these days. For dormant threads.
I would like to highlight that I lock old threads, based on the last post date, not the thread start date.
3/6 months can be a bit too long and too short depending on the threads section/context. Safest bet is to use a year (plus its far easier for me).
I haven't yet had any suggestions/replies for specific forums to have longer/shorter dormant thread locking policies, except for the Programming Forum (Hooman - never lock them, as they get added to after extended periods of time) & The Main Projects (Under control of sub-forum Mods - So they can control idea flows)
Edit: See here for the others:
http://forum.outpostuniverse.net/index.php?showtopic=4230I'm glad this thread didn't get locked. I think that there should always be at least one thread open in every forum/subforum.
I can't remember where, and nor can I be bothered to search for the post/thread. But it has been generally accepted that if you want to continue to comment on an old locked thread you start a new one and reference, with a link, the old one. If a Mod deems the new thread to be a continuation of the old one it can be merged. The majority of the time the new thread is at a tangent to the old one and therefore not merged.
Edit - Added reference to my Spring Cleaning post, couple of grammatical/spelling errors I noticed