Exchange does not clear out user's deleted items folder - what you're looking at is likely how often it clears the "dumpster" folder internally I'm guessing. This does not clear the individual mail uers's deleted items folder.
You can check out the Message Store Manager tool (Microsoft has a similiar tool in their SDK) on its home page here:
http://www.ciscounitytools.com/App_MSM.htm
This will let you setup various rules for agents that run on schedules you set - so it can clear deleted items folders, age read messages, delete non voice mail items etc... you may find it helpful.
