to archive threads in your collaboration systems, you need to create a workflow.

1) add new workflow:

object type:none
Title Put a name here that identifies what this activity is doing. Description Put a long explanation here of what this activity is doing.

2) add the workflow activity named "archive old threads" This workflow activity will go through all Collaboration Systems (CS) in your site and, based on the settings of that CS, archive old posts.

3) Remember to add this new workflow to the scheduler. once a month is probably frequent enough. or even once a year.