If you are in a group details, to which you have access by clicking on the group name in the left hand menu panel.

You may now go to the group settings which are located in the upper right of the screen and click on Auto archive settings.

A modal window will pop up where you can set auto-archiving, of conversations in the group, ON or OFF. If it is ON then number of days field is displayed where you can set after how many days of inactivity the conversation should be archived.