Fritts's rule of thumb? "If you even slightly debate that a pair looks or feels worn out, you're right!" A great way to test-drive her theory is to give your older pairs a feel. If you happen to have stretched-out underwear, that's a surefire sign to replace it. You'll also want to look out for visible holes, stubborn stains, sagging, and unravelling seams.
According to Griffiths, you'll begin to see changes in fit, fabric, and colour the longer a pair is worn, which is why she adheres to a more reasonable underwear shelf life of 12 to 24 months. "The fabric will start to stretch out and lose its shape, and the colour may start to fade," she told POPSUGAR. "You might also notice some discoloration in the gusset." Disclaimer: this is normal, due to the natural PH found in sweat and discharge.