If you want to change the default period of item retention ( Recover deleted items) from 14 to X days. To specify a value, enter it as a time span: dd.hh:mm:ss where d = days, h = hours, m = minutes, and s = seconds.
For example, to specify a 15-hour interval, enter 15:00:00. The maximum length of time to retain deleted items is 24,855 days. By default, deleted items are retained for 14 days. This attribute applies to all mailboxes in this mailbox database that don’t have their own item retention attribute set.
Through the EMC:
Go to the properties of the database->limits tab->”Keep Deleted Items for (days)”.
Set-MailboxDatabase "YourDatabaseName" -DeletedItemRetention 7.00:00:00