I know this is probably too complex to implement. But I'd say:
* 1st post in a thread can always be edited by the author.
* The current last post in a thread can always have additions appended. (This allows for adding stuff, instead of making multiple sequential posts if no one else is posting.)
* Other posts, can be edited for up to two days since posting. Sometimes people post and don't come back for a while, then realize they screwed up quote brackets or something. (Me, for instance.)
It's good manners to mark any substantive edits with "Edit, because ..."
But trying to enforce that would be a whole can of worms.