I would suspect, the more you use a clay bar, the greater the chance you will mar your paint up. That no matter how many times you fold, knead, the bar becomes so loaded with contaminants, all you are doing is spreading this all over your paint, glass, etc.
Sadly, clay bars today seem to cost a Kidney, and as for the Nano Sponges, I have two, and I think they are total crap. I cannot get them to work like clay does, life is too short for me to spend an hour on just the hood, and using a 1/2 gallon of lube to see very minor improvements.
That I'll say the clay bar still reigns supreme in my book. Just don't drop them, because you just flushed that money right down the toilet.
I'd say one use, maybe two, then toss that Bar, open another. It of course depends, but on average, I'd say anytime you are resorting to a clay bar, you vehicle is probably quite monkeyed with crud.
See the vids with Mike Claying? I'd say in those instances, one use, then toss.