I don't think there is any time limit for this. Any thread can be discussed when it comes to one's attention. It is a fact that those who join recently will be seeing the threads posted long back. Naturally if the contents of the thread has still value then one can react by responding.
Certain threads will be just based on issues or events occurring during a particular time, which ma not have relevance later, hence no meaning in responding.
Gold Member, SPK