Managed Mailbox policies in Exchange 2007…when do they take effect?

I made a managed mailbox policy in Exch 07 that is set to delete all mail items older than 90 days in the entire mailbox. If I apply this policy to a user, how long until they notice that all of their old items are gone? Do they get a warning? When will it run?

Do you guys know of another, perhaps better more straightforward way of doing this? Like have it run every Saturday and let the users know. Thanks for the help.