HA,
That still makes no sense, Anuradha, I think you are asking this?
Can you have a policy on which a user decides how many items get archived per night? The answer is NO.
If I was you and Exchange space was an issue, I would consider upgrading to Enterprise Vault v8, in v8 you can do age and quota based archiving, which will massively reduce people reducing people receiving over quota messages as well as reduce your information store.
--wayne