It should last at least 1 CX race or much longer.
I didn't see this thread last year, but using an old tube with removable core road tube up is the hallmark of Ghetto tubeless, especially when paired with Gorilla tape and non-tubeless ready wheels. I typically kept a good 1 inch of tube around the valve stem. Worked for me for a couple of years on my stock Specialized wheels with non-tubeless-ready Ignitors.