I have a pair of Williams-Sonoma oven mitts that I've been using for everyday cooking for about 3 years. Yesterday I went to pull out a baked potato from a 400F oven, and it burned me right through the glove. Potato went flying.
I didn't realize my oven mitts had worn out until this unfortunate incident. I think in the past, I've always just tossed them when they got irreparably stained. This pair stayed much cleaner than my past oven mitts, so it didn't occur to me that they had reached the end of their useful life.
When do you know you need to replace your oven mitts?