I only get three track weekends on a set and that's really one too many. I should dump em after two, but because I'm cheap I usually push it into a third session where they've clearly lost most of their grip.
I think there are two *wear* issues involved: compound behavior and tread life. Hoosiers are pretty well done after 16 heat cycles, they may have tread left, but the tires don't stick. I usually hit the compound limit first (but there's not much tread left at that point either).
Total waste of money for the street, plus you'll probably never get em hot enough to notice any superiority over street tires.