It should activate quickly. I know precheck took about 2 days total for me from interview to usable. But I don’t have Clear so can’t say if it does actually go quick.
That being said, clear may not be really worth it alone. I just went through MCO a few days ago and waited all of 30 seconds in the precheck line, and the normal line was only about 10 minutes. Shutdown doesn’t seem to be causing trouble there.