What is the average approval time for a TS-SCI National Level Interim clearance?

It all depends on the situation and the needed investigative scope. If your grandparents live in China or RU and your parents were born there... could be longer than the guy who family never moved out of Kansas City.

I’ve been in the open investigation process for about 8 months now. My training NCO recently just put in a request for an interim, and I may be getting it in a month, or so.
I hope this gives you an idea of the average length it can take.

Varies. Interim SCIs are done by Army CCF - not locally. Once the NAC portion of the T5 is complete, plan for 6 months after submission, but I’ve seen them go faster. Talk to your S2.

Currently, Interim TS/SCI clearances are only approved by the DoDCAF. This is due to a number of factors that changed the process over the years. Local CDRs and approve Interim Secret and TS but the Interim SCI portion takes time.

18-24 months is the norm. Only less if system is not backlogged in your area.