When you go to view a topic, it has to search through your TOPICS table to find the topics and search through the REPLIES table to find the replies for that topic. The more data that is in those 2 tables, the longer it will take for it to search for these topics and replies.
So the archiving feature moves some of the topics and replies from those 2 tables and put them in another table A_TOPICS and A_REPLIES. This helps to keep the TOPICS and REPLIES table smaller which makes the queries find the topics and replies faster.
Archiving is only needed when you have a large amount of topics and replies on your forum. Or if viewing topics seems slow.
Depends your database wii. Access database's can handle only so much. MS SQL and MySQL database's can handle a lot more data without any decrease in speed or overhead.
It's not really a fix number of posts or replies that you look for before you archive. It's the performance. If your topic.asp page is loading slower than usual and you have a lot of topics/replies then archiving would be a good idea. The forum will just search through less data when it is retrieving the topics/replies you are viewing.