Once the items are deleted permanently from the outlook or outlook web app, they go to Recoverable Items >> Deletions Folder, These Items are retained there for 14 days. we can increase this limit to maximum 30 days, If you wish to retain items more then 30 days you can put the mailbox on litigation hold for that time.
Set the limit for a user.
Set-Mailbox -Identity "Sunil Chauhan" -RetainDeletedItemsFor 30
Set the limit for all the user Mailbox.