I had two plugs in a tire, put them in myself btw, and when I was ready to get a new set of tires, all four actually, I burned them all the way down. I put these plugs in when the tire was nearly new, had 99%+ tread, I did many many burnouts, launches etc etc, and just generally abused the hell out of them at the end. They didn't leak a bit, and I drove on them to the tire shop to get my new tires installed. They were nothing special, Kelly brand.
I've never had a problem with a plug, but I guess it's different for everyone maybe..