How long will Microsoft retain a mailbox on Office 365 after leaving a user un-licensed?

Microsoft will retain the data on a mailbox for 30 days on an unlicensed user account.

TIP: If you need to retain the data for an unlicensed user you can convert the user into a shared mailbox and then remove the license from this account.

As long as this account doesn't grow over 50GB, you will not need a license for it.

If you need to reconvert a shared mailbox into a normal user account, make sure you have a spare license assigned to the account before starting the process.