Nov 23, 2015 - 05:30 AM
Inbox / Draft / Sent = 90 days default unless you manually change it in settings (not recommended)
Junk = 45 days
Trash = 14 days
Custom Folder = Never deleted
Once the emails are deleted, they are not recoverable.
If you need to store your email you can download them via iMAP and then save them as a backup