How old can your ETS/Retirement orders be before you can no longer invoke your rights covered under the Servicemember Civil Relief Act?

Unfortunately, the Servicemember Civil Relief Act does not cover former servicemembers.

That said, the landlord would probably still accept an ETS or retirement order with the understanding that the person needs to move back to their home state, and if they don;t work something out, the landlord could be looking at an abandoned rental anyway.

Section 3955 of the SCRA, 50 U.S.C. § 3955, addresses the topic of lease terminations.
https://uscode.house.gov/view.xhtml?path=/prelim@title50/chapter50&edition=prelim
It only covers while in service, addressing PCS and TCS.

Google search:

'Your protection under the SCRA begins on the date you enter active duty and generally ends between 30 and 90 days after the date of discharge from active duty"

"If you have questions about ending a lease early, contact your installation legal services/JAG office. You can also find the closest office with this Armed Forces Legal Services locator."

https://www.militaryonesource.mil/military-life-cycle/deployment/preparing-for-deployment/military-clause-terminate-your-lease-due-to-deployment-or-pcs/#:~:text=Your%20protection%20under%20the%20SCRA,of%20discharge%20from%20active%20duty.

It also depends if there's a military clause in your lease (which you should have looked for before signing the lease).

Usually when a soldier leaves the military, their orders include transportation of household goods to the location where they are going to be living as a civilian. From what I have understood, it does not need to be at your current location.

Q: What if I need to break my apartment lease because of my military service?

A: While in military service you may terminate, without penalty, residential and business leases that
you entered into prior to joining the military. You may also terminate residential and business leases
entered into while in military service if you receive orders to deploy for a period of at least 90 days,
or orders for a permanent change of station. To find detailed information about how to invoke your
rights under SCRA, contact your nearest Armed Forces Legal Assistance Program office. You can
find the nearest location at http://legalassistance.law.af.mil/content/locator.php.

I think what you really want to know is how long after separating from service may the SCRA rules be invoked. If you are asking for yourself and you are still on active duty, you should take this question to your legal assistance officer (JAG) for an authoritative answer. Because, the answer can vary a lot depending on circumstances. If you are counseling a soldier, send them to JAG. Meanwhile see this, item No. 5 in particular.
Good luck.
https://www.justice.gov/servicemembers/servicemembers-civil-relief-act-scra