A few signals to watch for. The top wicking layer can start to feel damp against your skin instead of dry. The gusset feels heavier or a little bulkier as fluid collects. You might see traces of blood on the inner layer or near the seams. And once a pair has been on for a while, you'll often notice scent before anything else, since trapped fabric can carry odor before leak protection actually fails. As a baseline, most brands suggest swapping every eight to ten hours, sooner on heavier days. When in doubt, change. Sitting in a damp gusset for too long may feel less comfortable against skin over time.