How long does a CPI instructor certification last?

I believe it depends, in most of the cases, on the institution issuing the Cert.

Although some Certs and their validity are controlled by the regulatory or standardization bodies, mostly in my experience it is the institution that issues that are deciding how long to issue it for.

It may have various reasons though, e.g. industry development speed, regulatory or procedural updates, etc.

And you can always add some more certs to your portfolio by taking some free courses on EntirelySafe.com
Hope this helps