Hey there!
For quite a few forums ('forums' in this case meaning Emulation and otherwise, forum-wide), the revival limit is one month. In some other cases it happens to be two months.
At the top of each sub-forum, there is a thread that will outline the rules of that sub-forum. These threads usually outline whether the revival limit applies or not. Off the top of my head, the Toolbox forum is the only one that's revival limit is two months - all others are one month, if the limit applies at all. :)
Obviously, there's no need to close any and every thread that passes the mark: when someone wrongfully posts in a thread that has passed the limit, you should report that post, and a moderator will close that thread. Just in case you were wondering!