OK, how long does a rear tire last? I was cleaning the bike up and looking at the tread blocks. The leading edge is now worn to ~3/8" radius and the trailing edge ~1/4" radius. Not sharp square lugs anymore. Still soft rubber from poking it with a fingernail. I am sure upper class riders would have replaced it long ago. But what about a lowly Novice rider? At what point should I consider replacing it?