What do your current policies and procedure's state? If noting exists, good on ya, you get to create one and customize it specific to business needs, yeah!
If noting exists, or you see gaps in the process, look at ITIL, ISO, NIST, and even PCI, and others for their recommendation on retention of terminated user accounts. Use the longest retention time (of those suggestions above that are specific to your industry, eg. if you don't process credit cards, don't sweat PCI) to your policy; I would guess that 90 days is the answer.
Term accounts, as others have stated, append something to the account to denote the status visually and in your user management tools. You can do it to the display name if you are concerned about orphaning objects, and items the user created on various federated tools.
Upon disabling, you should migrate their data to some type of secure filesystem, let the manager or the individual who would become the data owner to review the users digital artifacts. Add that to your policy and how long the manager/data owner has access before it is archived to tape and then eventually deleted.
You could delete the email account from the user object. Some people use corp mail as a freaking file system which can be annoying to backup. You could consider putting a forwarder for their manager for 90 days as well, whatever works for your business's needs.
Part of the procedure should include an "account/user term checklist" for termination that has to move through the IT, Ops, Sec, Mgmt, and HR teams, this should be signed by each manager once their respective responsibility has been completed.
This is FAR from a comprehensive list, simply some things I have used in the past. Moreover, I want to provide you with suggestions to focus you back on your CISSP knowledge and to give you some places that you can create policies that would pass third party auditing and to empower you to improved your new term process.
Always, refer to your policies and procedures, lacking them, document and get approval from stakeholders and your CISO and maybe the legal team first.
Finally, data can bite in the posterior, if you keep it too long! This is especially true if the sector of industry your business belongs is highly litigious.
Best of luck!
