Deleted Messages Disappearing

Deleted messages are disappearing after about a week.  I don't want them to disappear ever.  Sometimes I need to look for older messages.  How change I change the number of days to keep old email to "never"?  I simply want them to remain in the "deleted" folder until I want to delete them permanently, not when Outlook decides to delete them.  As a professional writer, this has caused extreme problems with editors and it must be fixed.