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

Interims are usually granted locally. When I was a security manager in S2 (and it's been a few years since I have done it) interim clearances were granted by the installation security office after reviewing the SF86. They did deny some but that doesn't mean the final won't be adjudicated and granted. I saw people get denied an interim and get a final clearance.

It should not take that long at all to get an interim as that is only to give you temporary access until your investigation is completed and adjudicated. You need to go talk to your security manager in S2.

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.