Unity has no retention policy - it will never delete messages unless asked to do so. When you delete messages over the phone they go into the deleted items folder or are deleted entirely based on a Class of Service setting for that - however Unity does not "age" messages automatically based on rules.
You can use the seperate Message Store Manager tool which you can get here:
http://www.ciscounitytools.com/App_MSM.htm
this lets you setup mailstore rules for users. Microsoft has similiar (though less robust) tools for doing the same type of things that ship with Exchange 2000/2003.
