Well, it won't help you now of course :)
The idea of Single Item Recovery is to ensure that you can recover items a user deletes even if they empty their mailbox "dumpster"
Personally, I prefer 30 days for each mailbox, but that is really up to each business and their requirements.
I wouldn't rely on that however when using the -DeleteContent switch. Instead, copy the items to another mailbox before removing them with that command
Example from that doc I linked above:
Search-Mailbox -Identity "April Stewart" -SearchQuery 'Subject:"Your bank statement"' -TargetMailbox "BackupMailbox" -TargetFolder "AprilStewart-DeletedMessages" -LogLevel Full -DeleteContent