How long to retain terminated users in AD?

Is there a “best practices” guideline anywhere out there? I work for a construction company with about 400 users, and of course we’ve been going through large layoffs. I’m taking over some day-to-day management of AD and we have a disabled users OU that has hundreds of old employees.

I’ve asked around the IT department and no one knows why we keep all of these users. I know we want to keep their mailboxes for a few months, but will deleting them from AD delete them from Exchange?