There may be situations where you don't want to keep all the discussion messages or group instant messages in a space for ever, perhaps to conform with a company retention policy.
If you are a system administrator, you can set the maximum ages used by all spaces by going to the system settings, choosing the Group defaults category and then the Limits and quotas category. Under the Retention section you will find settings for the Maximum age for an instant message and Maximum age for a discussion thread.
To change this for an individual space, go to Settings, choose the Advanced settings view and then the Limits and quotas category. Note that by default group administrators do not have access to this category. If you want to allow them to change these settings then you will need to change its permissions - see How do I prevent group administrators from changing certain group settings?
Note that both these settings will only delete threads of instant messages and discussion threads that haven't had new messages added to them for the number of days specified as the age. If they are still in active use with new messages being added then they will not be deleted.