When you loose confidence in it.
Not to be confused with simply needing a relace. I had a teammate whose glove busted a web lace midway through a game. I saw it in the dugout and pointed it out. He had missed it. THe laces were all so old and brittle, that when I later relaced it, the lace came off in pieces as I pulled them out. Catching at the time, had he taken one more throw from the outfield, the ball would have gone right through.